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.