1. Arcana Evolved (Malhavoc Press)
An absolutely superb, 100% compatible with D&D combination campaign setting and alternate PHB, from one of the creators of 3e, Monte Cook. If you don't use the world, it's like getting four Races of and four Complete books rolled into one (with 25th level progressions instead of 20th!); if any of the local DMs do use the world, it's all that plus a campaign setting.
Alternates: Accept no substitute. It's an expensive book, but again, it's like getting 4-8 splatbooks and a campaign setting for the price of two splatbooks. A no-brainer.
2. Expanded Psionics Handbook (Wizards of the Coast)
Probably the best "magic" system ever released for d20 - certainly the best of the optional systems put out by WotC. There's nothing in the XPH not to love; it's crunch is wonderfully handled and usable by all characters, it's flavor is solid but not so overpowering you can't remove it if you don't like psionics in your fantasy, and, as with most major WotC releases, its production values are excellent.
Alternates: Accept no substitute. Wizards' Tome of Magic and Magic of Incarnum both cover the non-core magic system niche, but not as well. There are 3rd party takes on psionics that are more 'psionically flavored' - but aren't as superb from a system perspective.
3. d20 Modern (Wizards of the Coast) or Grim Tales (Bad Axe Games)
d20 Modern is more focused on modern roleplaying, Grim Tales is more of a toolbox with a slight pulp slant. Both are excellent second cousins to D&D with some great design work and many elements that can be used on their own or incorporated into D&D.
Alternates: If you actually want a modern roleplaying game, consider Spycraft 2.0 (Alderac Entertainment Group). It's an excellent game in its own right, and has great tools for DMs, but is not as compatible with D&D. For grittier pulp adventure, consider Conan the Roleplaying Game (Mongoose Publishing) - again, not as compatible.
4. Mutants and Masterminds (Green Ronin)*
Presented as a Supers RPG using a variant of the d20 system, Mutants and Masterminds is actually an amazing toolkit product that can run any genre. It only gets bumped to fourth place because it isn't really compatible with D&D, but it's a d20-based toolbox game if your local gaming group ever wants to try non-fantasy. An important addition to a 'library' type setup, IMO.
Alternates: Accept no substitutes. Nothing even comes close. If you want a d20-based toolkit, you want Mutants & Masterminds. The only other option would be a completely non-d20 game such as HERO (Hero Games) or GURPS (Steve Jackson Games).
5. Dragonmech (Goodman Games)
A great steamtech/fantasy mecha setting with ever better rules. The coglayer class is a must for any tech-type character in your games, the mecha rules are playable if unspectacular, and both the setting and crunch are modular enough they can be plugged into an existing campaign without disrupting it.
Alternates: For mecha, consider d20 Mecha (Guardians of Order) or d20 Future (Wizards of the Coast). For steamtech/steampunk, consider the Iron Kingdoms Character Guide (Privateer Press) or OGL Steampunk (Mongoose Publishing).
6. Rokugan Campaign Setting (Alderac Entertainment Group)
Whether you're interested in the setting or not, this is another 'excellent class book masquerading as a campaign setting,' albeit not on the level of Arcana Evolved. The samurai here is much better than the one in Complete Warrior and the Ninja a very different take from Complete Adventurer, but the Shugenja is largely unchanged. On the flip side, the Courtier is almost a requirement for a non-magical social class, the Inkyo is an interesting monk variant, and the selection of feats and prestige classes is very good.
Alternates: The reason I place this so highly is the Courtier class; the rest is good, but the courtier fills an otherwise empty niche. You could grab the Player's Guide to Arcanis (Paradigm Concepts) for its Patrician class instead, or, for a weaker take on the concept but the strongest book overall, the Dragonlance Campaign Setting (Wizards of the Coast) with its Noble class.
7. Monsternomicon (Privateer Press)
Certainly the most flavorful monster book around, and also one of the best. It's designed for Privateer's Iron Kingdoms, but most of the monsters can be used anywhere and to great effect.
Alternates: Monsters of the Mind (Green Ronin) is a very good supplement of psionic monsters attached to the Mindshadows (Green Ronin) setting. Warcraft Manual of Monsters (Sword & Sorcery Studios) has wonderful creatures, but is difficult to find. Bestiary of Krynn (Sovereign Press) is an excellent Dragonlance resource with very high production values, but has a new, revised version coming very soon.
8. Lords of Madness (Wizards of the Coast)
Pure personal preference to select this over Draconomicon and Libris Mortis. Lords of Madness is Wizards' book of aberrations and gives in depth information about beholders, illithids (mind flayers), aboleths, grell and neogi, as well as a new race, the tsochar. It includes decent PC options, excellent monsters, and a lot of campaign ideas.
Alternates: Draconomicon (Wizards of the Coast) if you like dragons better than aberrations. Libris Mortis (Wizards of the Coast) if you like either undead or bad Latin better than aberrations.
9. Eberron Campaign Setting (Wizards of the Coast)
A very creative newer campaign setting with a lot of great ideas and support materials. The races are interesting, the Artificer class likewise (though, more than anything else on this list, the artificer is easy to get overpowered without really trying - DM beware!), great use of the action point rules, and some very cool antagonists.
Alternates: Forgotten Realms Campaign Setting (Wizards of the Coast), Iron Kingdoms Character Guide (Privateer Press), Dragonmech (Goodman Games)... a campaign setting book depends heavily on what you're looking for, but for the actual setting Eberron gets my pick.
10. Blood and Fists (RPG Objects)
Nothing less than the best martial arts supplement on the planet, at least for d20. Design for d20 Modern, Blood and Fists works perfectly for D&D, too. A must if you want to run a martial arts heavy campaign.
Alternates: Beyond Monks (Goodman Games) is a very good fantasy-specific treatment of the subject - think of it as Complete Monk, in WotC terms. Martial Arts Mayhem (Game Mechanics) is another very good Modern take on the topic. Oriental Adventures (Wizards of the Coast) and Rokugan (AEG) both have lots of martial arts mixed in.
11. Oriental Adventures (Wizards of the Coast)
THough it's a 3.0 product and covers much of the same ground as Rokugan, this is still an excellent book. Great classes (shaman and sohei aren't available anywhere else, nor are some of the PrCs, such as the highly flavorful and fun blade dancer), races and monsters. If you get this, also try to pick up Dragon #318, which had the official 3.5 update.
Alternates: Nyambe (@@) covers a less well-traveled 'exotic' setting: fantasy Africa. It's 3.0 with no readily available update, however, and can be hard to find.
