I picked up Eberron the week it came out, and Unearthed Arcana because I was bored and was in the vicinity of a gaming store. Never do that. I don't NEED a book of variant rules, yet I have one.... but I have used it in some design stuff, so it works out.
My roommate buys more books than I do. I have access to, from that list, the Expanded Psionics Handbook, and the Planar Handbook. I plan on picking up maybe Monster Manual III, and the Sharn book (because Sharn rules).
I really don't go overboard with gaming purchases.
-Matt