My two most recent purchases are:
D&D Essentials Dungeon Master's Kit and the Deluxe Dungeon Master's Screen - I've decided to give 4e another shot, and Reavers of Harkenwold has been getting a lot of very positive reviews. We resumed our 4e game (which has been on hiatus since December when we finished The Slaying Stone) and the first session of Reavers went pretty smoothly. I've always been fond of minis, but I used the included tokens exclusively for all of the enemies and that actually sped up combat quite a bit, eliminating all of the questions about which figure represents which opponent and only having to write down token numbers on my own combat record.
It's a really nice set, and even though I have both the DMG and DMG 2, the DM Book that comes with the kit is easier to read on public transit and takes up less space on the table. The Deluxe Screen is nice too, and I hated the artwork on the original one. Didn't actually use the screen much, but it hides my notes quite nicely.
Back in January, I picked up an extra copy of the Player's Guide from the Mentzer Basic Set and a near-mint copy of B5 Horror on the Hill from the used section at an FLGS. The module looks absolutely brutal (and it has that reputation), but it was nice to have the 2nd player's book for the Basic game that we played a few months back. Might still go back to it if 4e doesn't pan out for us.
I got a bunch of D&D stuff for Christmas, which fills the 5th slot with: D&D Essentials Monster Vault (awesome), Dungeon Tiles Master Set 1: The Dungeon (also great, as I missed these sets the first time around), and a Chessex battle mat (I've resisted them for years as I don't like the marker-on-rubber mat look, but it's just a lot more flexible than the alternatives).