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GURPS Conan. I've read through it quite a bit, and it's inspired some great ideas, but I've never actually put it to game use (of course, not playing GURPS could have something to do with that )
Gary Gygax's Dangerous Journey's (esp Mythus Magic).... just never got around to learning the system.....although that is still one of the best magic books ever produced....
and Mage:The Ascension (Do all of us own this.....do none of us use it), Mummy:The Resurection (not a great book....but mummy's running around in modern times is still one of the funniest things i can picture)
In Nomine, loved the idea of the game, bought it years ago, and have never got a chance to play it. The fact it has probably the worst design for character creation, ever (almost randomly scattered through the first half of the book, with no indicatiosn of where to find what bit of info) probably didn't help.
Monsternomicon -- I love this book, but by some weird coincidence, I haven't actually used anything from it yet.
Manual of the Planes -- I don't know that I necessarily will use this, unless I have a completely planar game, but it's still one of my favorite RPG books.
I thought about saying Oriental Adventures and Rokugan but I actually have used bits and pieces of them here and there.
- Terra Incognita. It's a shame, really, because the setting appeals to me, but Fudge doesn't appeal to my players...
- Lord of the Rings. Not quite sure if it's actually good, but I'd love to try it.
- Brave New World. If only I could find some players who'd like to play in that genre (btw., that's the reason I've stayed away from Mutants&Masterminds and the Marvel RPG for now... or otherwise they'd probably be on this list too).
- Mutant Chronicles. Love the world and wargame (Warzone), but no one else does.
As for d20 books:
- The Book of Taverns. Hadn't had the chance to include one of those in my homebrew. But that might change in the future, although I'm not sure yet.
Tie between Monsternomicon and Tomb of Abysthor. I love both of them, but I have been a player for the past year, and have not been able to run more than one shots, so not much use for either of these right now.
The best book I've owned but NEVER used, would have to be Lost Souls, from Sage Lore Productions.
It was the ORIGINAL game where you played dead people!
Think of the movie Ghost with Patrick Swayze, and you have the basic premise: people trapped between death and life who search for karma to finish their journey, and who possess power over the physical world in many different ways.
You could also play a medium who helped these ghosts in their journeys. It was a very cool concept, but with a failure on my part in getting any players interested in it.