D&D 3.0 & 3.5
After spending years using some of the same books repeatedly, I've decided to offer quick reviews and highlights of some that might help you. Although they're out of print and older now, they're still on my shelves for a reason: I know I'll come back around to them again.
The Player's Handbook II can be useful for players, but it also provides some good options for Dungeon Masters to take advantage of. The character background and archetype section can be a great quick reference when creating characters (roll a d10 in the first section for a random background, and/or play with matching backgrounds and personalities for some interesting results). The section on affiliations can be a fun way for the PCs to create their very own official group with specified benefits and obligations, and the details can help the DM keep the power level in check. For long-term games, the retraining section can help to undo poor choices made months or even years ago in an organized fashion.
The environmentally-based Frostburn and Sandstorm are fine references, even if you're only using them for short stints through extreme temperatures. Not only can their information work for a basic fantasy world (or at least regions within that world), but the additional dangers can make great changes of pace for planehopping campaigns. There are also unique monsters in both books that can go a long way in setting the tone of desolate wastes, whether they're freezing crusts or burning under an unforgiving sun.
Seafaring campaigns can be a lot of fun and the Monster Manuals have aquatic creatures listed, but the details needed for spending a lot of time on a ship were reserved for Stormwrack. Rules for maelstroms, naval combat, and navigation are spelled out, as well as shipworthy applications of the normal skills. The maps of various vessels can help set up your battles nicely, too. The selection of seabound creatures adds more possibilities to the relatively limited number found elsewhere.
After spending a lot of time in the wilderness or in underground tombs, it can be difficult to bring a campaign back to civilization and figure out what to do in a locale without dragon lairs. Cityscape has support for the city as its own dungeon or as a collection of potential dungeons, but it also takes a look at cultures, districts, governments, and organizations. The section dedicated to running the city environment (covering things like town history, race relations, and crime) is admittedly brief but at the very least the book can spark ideas, even if you're not interested in mapping out your city district by district.
If undead have become old hat or you haven't used them in a while and you'd like to beef them up, Libris Mortis can help. I recall some of the feats being rather useful and more creatures are always good. It also has some monsters broken down as monster classes, in case you wanted to run an undead campaign using the monster classes from Savage Species. (While I was never quite thrilled with the execution of that concept, I could appreciate what it was trying to do for making nearly any race playable even at low levels.)
The Forgotten Realms 3.0 Campaign Setting book takes decades of classes and lore and condenses it beautifully into an easily referenceable Bible of the Realms. I have spent years in awe of how well this book was put together, not only for navigation but also for inspiration. The differences of the Realms are outlined in classes, feats, equipment, and languages, and the richness of the history is apparent from the first pages without being utterly overwhelming. The clerical domains of the Realms are awesome, many of the feats are neat, and the details of the continent are easy to use or ignore, as needed. The quick summaries of Faerun gods and history in the back of the book give enough to run with, but excellent resources cover both aspects in greater depth later on.
Faiths and Pantheons is one of the keys to a rich and rewarding Faerunian experience, particularly if you enjoy the variety of deities available in the setting and/or if you enjoy details that will bring different religions alive in your game. Each entry gives history, dogma, and details about that god's worship, from laypeople to high clergy. I've always been partial to the art and the holy symbols, and the prestige classes in the book are worth their weight, by and large, which is difficult to say of most books. I've never wanted or used the stats given for each of the major gods, but if you wanted to have them for an epic-level, god-fighting campaign, they're there.
Races of Faerun is a nice support piece for playing in Faerun, since it gives more information on the various races and what makes them different from each other and from the standard races.
Magic of Faerun can be useful even if you're not playing in the Realms and the villainous organizations in Lords of Darkness could make appearances in nearly any campaign, with just a little alteration. Each group comes with an encounter and a related map, too. If you're into the lore or if you're a fan of the video games, Lords of Darkness gives even more information about old favorites.
As far as I've seen, Silver Marches is the most compelling of the regional supplements for the Forgotten Realms, even though it's the only softcover in a handful of hardcover regional books. I can't help but think that Ed Greenwood's influence might have made a vital difference and given it that extra touch of loving elaboration (which was reined in nicely before it became tedious). It's packed full of vivid details about the land, the settlements, and all the dangers in between. The pull-out map is decent but not exactly necessary.
Lost Empires of Faerun helps Dungeon Masters access the considerable history of the Realms in a more direct and engaging fashion. It starts with the history, as much of the Forgotten Realms books do, but moves on to sites and enemies. As far as I'm concerned, one of the greatest attractions of Faerun is the sense of all that's gone before; this book helps you run things from the ancient past.
The Grand History of the Realms is a more distant kind of resource, if only because it provides so much information that it's printed in smaller type in many places. If you want to run in earlier eras of the setting, this book is a must. It has some good maps taken from previous ages and list of dynastic rulers of major kingdoms. But even if you don't think you'll use much of the history, you can mine the events of the past for ideas about the present and future.
Gary Gygax's Extraordinary Book of Names is a gem for gamers since it was created with roleplaying in mind. It begins with a lovely, lengthy section based in real-world names from across the globe, as well as during medieval and ancient times, with male and female names delineated clearly. Each subsection starts with a few paragraphs about naming practices, honorifics, and pronunciation. The first part of the book alone will ensure its usefulness in many games for years to come. There are also sections about epithets, titles, groups, and place names. The chapter on fantastic names has lists of fantasy names divided by syllables and finishes up with a collection of generators that provide even more mileage. If you love character names, try to find a copy of this book. You will not be disappointed.
Relics & Rituals: Excalibur is a Sword & Sorcery 3.5 offering that does a very good job of developing a fantasy Arthurian setting where you can play hobgoblins along with adjusted versions of the other major core races except gnomes. It does rely on tropes and a power level that are different from the norm but once you catch on, it's not hard to extrapolate. If you're looking for itemized medieval clothes, some unique spells (like detect fey and selective silence), some useful magical items (like fleet armor) and minimal cutting and pasting from the SRD, you're in luck. We've used bits and pieces from the book in many other campaigns, and the art is lovely throughout.
Ultimate Campaign is a Pathfinder resource that can easily be used for other fantasy games, particularly other d20 campaigns. While it has some handy things for players (like the extended background generators divided by race and class, as well as a ton of traits), it offers a lot for DMs. It has an organized way to handle downtime events, like running a business and constructing buildings. While many groups might not want to bother with the in-depth system and bookkeeping attached to running organizations or kingdoms, a DM can draw a lot of ideas from the lists of events, complications in various life stages, and problems with maintaining a kingdom.
The GM's Aid line has all kinds of orderly, printable reference materials. I like the Condition Cards because they help keep track of all the little differences between states, like fatigue and exhaustion. I love the Monster Knowledge Cards because they break down which tidbits you give away when PCs make knowledge checks. I like to maintain some mystery if I can, and figuring out which secrets to part with in the heat of the moment is a pain that these cards eliminate.
The Player's Aid line is designed to help players keep track of the myriad details that can swarm around one character. I contributed a resource that helps reference bard songs from across a number of different books, but the line also has a treasure sheet, character sheets, and summoning cards. I can't tell you how many times the game has ground to a halt while players have gone digging to figure out what their summoned minions could actually do.
The Player's Options line adds considerable flavor to characters through things like flaws and loving details about different kinds of planetouched races. I worked very hard on the planetouched book to explore the diversity that's always been possible with aasimar and tiefling characters, while at the same time trying to balance them for ready use. While these books might not be everyone's cup of tea, they could inject a welcome dose of spice into your game.
The Book of Arcane Magic is a fine offering of spells, many with very practical applications. The sorcerer bloodlines and clockwork familiars are also neat. Bubo, here I come!
The Book of Divine Magic is another good collection of spells. While some are reprinted from other sources (like the much maligned Book of Erotic Fantasy, which nonetheless had some very practical spells), most are ones I haven't seen before.
Paths of Power is an integrated selection of new classes, skills, feats, and spells that support each other. Additional gear and guilds complete the experience. I have to admit that I'm biased where this book is concerned, since I was lucky enough to submit material to it. =)
Luven Lightfinger's Gear & Treasure Shop is a reference for all kinds of gear, which can get difficult to track down between books. It has armor, weapons, mundane stuff, instruments of all kinds, clothes, food, prosthetics, and a shop ready to go. While it might seem like too much detail to some, I like to have a book to go to for different types of cheese. Inserting these details at particular moments can make the world feel real.
Strategists & Tacticians: Offering a number of options for core classes, new classes, and maneuvers.
Inkantations is something I am glad I was able to help with, since it provides rules and specs for adding magical tattoos to your games, a concept I've always loved but haven't seen executed well before this. As a player, these kinds of options excite me; as a DM, these options amuse me.
Tome of Monsters is nice because you can never have too many monsters, and they go out of their way to tell you the real world roots of any creatures they obtained from folklore. There's also an eye toward providing support for familiars and animal companions based on the creatures listed.