Alzrius
The EN World kitten
Oh man, if we're going to talk about third-party books, then there are sooo many I could list.
Like, Dire Destiny Games' The Very Last Book About Alignment. Remember how, back in AD&D 1E, Gary Gygax wrote that you could chart a PCs alignment by assigning numerical values to various actions and charting the PCs' alignment fluctuations accordingly? No one wanted to do that, but this book does! It comes up with long categories of actions, assigns them values (which are presented in objective terms, but have different impacts depending on your existing alignment; a chaotic action can cost you several points if you're lawful, but might not make you that much more chaotic if you're already chaotic). It's a fascinating look at how to practically implement the nature of objective alignments in a fantasy world, and I definitely think anyone interested in that should give it a look.
Or Human Head Studios' Redhurst Academy of Magic Student Handbook. That was one of the most notable crossover supplements of the d20 era, presenting a magic academy that routinely teleported between different worlds (from different publishers), such as Nyambe (Atlas Games), Dungeon World (Fast Forward Entertainment), and even the Kingdoms of Kalamar (Kenzer Co.). It was all written from an in-character perspective, complete with notes scribbled in the margins by a separate person who was plotting the academy's downfall...
Or the entire Dragonstar line, from Fantasy Flight Games. It combined Spelljammer and sci-fi, putting your campaign world on the frontier of an expansionist galactic empire ruled by dragons, bringing new magic and high technology into your game. It really struck an expansive tone, doing a great job of making the setting seem like a genuine space opera as opposed to "sailing the high seas, but among the stars" that SJ has. Even today, a lot of people seem to remember it fondly, for good reason.
Or Octavirate Games' Exorsystems Inc., which while technically for d20 Modern, has a 3.X conversion appendix in the back. It's basically Ghostbusters with the serial numbers filed off, though it smartly introduces other groups that are competing with your characters (who've started a franchise in a particular area) in the same space, ranging from government agents to paranormal exterminators who eschew technology in favor of mysticism. Its method of translating Hit Dice and creature type into an in-character rating system is inspired also; it's definitely worth a look if there's something strange in your characters' neighborhood!
Or Anger of Angels, from Malhavoc Press (which later became Monte Cook Games). It's probably the single best treatise on role-playing celestials in the d20 System. Much like AD&D 2E's Warriors of Heaven, it gives statistics for those wanting to role-play a celestial, and has stat blocks for quite a few new celestial creatures as well! But the best part is undoubtedly the guide on the outlook, habits, and organization(s) of heavenly creatures. Also, don't forget to hunt down the web enhancement!
All that's just off the top of my head. There's so much good stuff out there.
Like, Dire Destiny Games' The Very Last Book About Alignment. Remember how, back in AD&D 1E, Gary Gygax wrote that you could chart a PCs alignment by assigning numerical values to various actions and charting the PCs' alignment fluctuations accordingly? No one wanted to do that, but this book does! It comes up with long categories of actions, assigns them values (which are presented in objective terms, but have different impacts depending on your existing alignment; a chaotic action can cost you several points if you're lawful, but might not make you that much more chaotic if you're already chaotic). It's a fascinating look at how to practically implement the nature of objective alignments in a fantasy world, and I definitely think anyone interested in that should give it a look.
Or Human Head Studios' Redhurst Academy of Magic Student Handbook. That was one of the most notable crossover supplements of the d20 era, presenting a magic academy that routinely teleported between different worlds (from different publishers), such as Nyambe (Atlas Games), Dungeon World (Fast Forward Entertainment), and even the Kingdoms of Kalamar (Kenzer Co.). It was all written from an in-character perspective, complete with notes scribbled in the margins by a separate person who was plotting the academy's downfall...
Or the entire Dragonstar line, from Fantasy Flight Games. It combined Spelljammer and sci-fi, putting your campaign world on the frontier of an expansionist galactic empire ruled by dragons, bringing new magic and high technology into your game. It really struck an expansive tone, doing a great job of making the setting seem like a genuine space opera as opposed to "sailing the high seas, but among the stars" that SJ has. Even today, a lot of people seem to remember it fondly, for good reason.
Or Octavirate Games' Exorsystems Inc., which while technically for d20 Modern, has a 3.X conversion appendix in the back. It's basically Ghostbusters with the serial numbers filed off, though it smartly introduces other groups that are competing with your characters (who've started a franchise in a particular area) in the same space, ranging from government agents to paranormal exterminators who eschew technology in favor of mysticism. Its method of translating Hit Dice and creature type into an in-character rating system is inspired also; it's definitely worth a look if there's something strange in your characters' neighborhood!
Or Anger of Angels, from Malhavoc Press (which later became Monte Cook Games). It's probably the single best treatise on role-playing celestials in the d20 System. Much like AD&D 2E's Warriors of Heaven, it gives statistics for those wanting to role-play a celestial, and has stat blocks for quite a few new celestial creatures as well! But the best part is undoubtedly the guide on the outlook, habits, and organization(s) of heavenly creatures. Also, don't forget to hunt down the web enhancement!
All that's just off the top of my head. There's so much good stuff out there.