Privateer Press:  they don't release much, but when they do; HOT DAMN!! they're good.  I'll buy anything they release sight unseen, even Warmachine books, which have nothing to do with roleplaying!  They also have really cool minis.
Green Ronin:  They don't always release stuff that is up my alley, so I don't buy sight unseen.  However, I don't have to worry about quality from these guys, at least.  The Book of Fiends series are amongst my favorite d20 books, and Mutants & Masterminds is an incredible game that we will probably have on a long-running rotation here in the very near future.
Fantasy Flight Games:  They initially released a lot of stuff I wasn't interested in, so I'm not swimming in FFG books, but the stuff I do have is all top-notch including Dragonstar. Midnight, the various Path books and their latest steampunk book.