As a companion to the "What's Your Favorite book of published GM advice ?" thread, this thread asks "What's your favorite GM accessory?"
Now, we're not talking reading material but actual tools you use to help prep for a game or actually use at the game table -- battlemats, white erase boards, 3D terrain, etc.
For me, the most useful GM tools are:
Pre-adventure prep:
•Brainforest (a now defunct outlining software for desktops & Palm Pilots)
At the table:
•MasterScreen - Customizable GM Screen
•The Game Masters Collection sound effects CDs.
•Chessex Battlemats; Their dice are cool too.