Aah, can you narrow down the genre? d&d, d20 modern, d20 sci fi... ?
Anyway my big votes:
sidewinder recoiled: spectacular wild west game, won some ennies, they have the main book in pdf (now published and printed by a larger company), and 2 smaller location books. very very very good stuff.
legends of excalibur: arthurian high adventure (that doesn't suck). lots of interesting rules, beautiful maps, too many ideas for me to decide what to do with it.
buy the numbers: I actually didn't buy this for me, I bought it for my math geek friend because I knew he wouldn't. He loves the mechanics behind the game, and was designing something similar of his own... I am also waiting to use this one in a game.
Hmm.. other ones to check that I won't type about because I have to sleep:
Darwin's World 2: Tons of pdfs, post apocalyptic.. awesome
Blood & Circuits: d20 modern gadgets & making smart characters cool.. awesome
that is all for now, good luck narrowing them down!