I buy about one or two books a month. I stick with pretty much WotC. Though I'll get the occasional third party book if it looks really good and has rave reviews.
I have some 1e stuff, tons of 2e stuff including all Dark Sun and Al-Qadim sourcebooks and boxed sets ever made. All the major Birthright material. The Dragonlance boxed set, TONS of FR boxed sets, some Ravenloft, almost all the Spelljammer stuff that ever came out. Almost every 3.0 and 3.5 book published by WotC. d20 Modern, Wheel of Time, etc.
Every Spycraft and SA supplement except African Alliance and Shop. Most of the SW d20 books.
And the occasional third party product like Conan, Nyambe, Dragonstar, etc.
I don't use most of it any more and my wife is pushing me to sell a lot of stuff. So I am thinking of selling some things on Ebay.
I also have a lot of HERO 5th edition stuff. But I buy that because I'm a fan. No one I know wants to play HERO.
I have tons of Rifts and other Palladium books. Am probably going to sell all those. I have the original Street Fighter RPG from WW and will probably try to sell that too. I have all the sourcebooks and the GM screen for it too.
I have tons of WEG Star Wars books, but I don't know if I can part with those.
I have cut down my purchasing a lot though. Now, I only buy WotC and that rare third party product (such as Lone Wolf d20).