Rules Compendium; maybe Races of the Dragon. That's about it for WotC. I bought MM5 and returned it - too many really weird monsters I'd never use. First monster book I ever returned (and I've bought alot).
Non-WotC...the last issue of Dragon, probably the Accordland books from AEG, the Alliance Player's Guide for the WoWRPG (I thought the WoWRPG was alot less compatible with D&D than it is)...some pdf stuff, probably.