In no particular order:
Spell Compendium - One place to look for spells as a DM is a godsend... well that and the PHB. I regularly read this book, it's great fun looking at all the new spell goodies.
GM Mastery Book I, NPC Essentials by Johnn Four - I love this book. I have read it twice and will read it again. It's full of many great ideas for campaign management, NPC design and roleplaying tips. It is really a great book, I will have to read it again for inspiration. If you like Treasure Tables, this is for you. Johnn, I hope you are seeing this... I want book two!
Combat Pad - This is a great tool for running combats. I used to use index cards, now everyone knows when their turn is coming up.
Status Cards - EN Publishing (I think) has these great cards you print out for various statuses. It's really helpful to hand out a card when a player is affected with something. I would add spell cards to this list but I have found that beyond 5th level they become unwieldly and I prefer a printed list.
Magic Item Compendium - Sure, it's full of unbalanced stuff, but you can tweak it all. It's got a lot of great stuff to give to the bad guys.
Honerable mention: Tome of Artifacts - I haven't read too much of this yet, but if you ever needed ideas for a campaign, there are about 30 of them in that book. Full of rich detail for every item, along with campaign seeds and rumors of items. Great stuff, good work to all the authors on that book, and a steal at the price point (it's a HUGE book)
*I would have said Dungeon and Dragon magazines... but there went the best gaming value ever*