I've tried just about everything. See bwlow for what the ratings mean.
Chessex Megamat (8/10); Dwarven Forge MasterMaze (9/3) and Miniatures (9/2); D&D Dungeon Tiles (7/5 -- 7/10 for con-going), Miniatures (9/10), and Fantastic Locations maps (6/5); Steel Sqwire Flip Mats (6/7 -- 6/9 for con-going) and Spell Radius templates (9/3); Steve Jackson Games' Cardboard Heroes (4/4 -- 4/7 for con-going); Dragonscale Counters (9/2); Green Dragon Studio's Dungeon Stamps (7/4); Alea's Magnetic Markers (4/2); Open Mind Games' Combat Pad (7/9); The Other Game Company's Index, Monster, and Spells cards (7/6); Paizo's Item and Magic Items cards (6/1) ... and I'm sure there's at least a half-dozen things I'm leaving out.
I've given each of the accessories above two ratings, with 1 low and 10 high. The first is "Concept," or just how "neat" an idea it is. In many cases, something with a high Concept score will be much more useful to other people than it has proven to be for me. (Steel Sqwire's templates is in this camp ... after using them for a while, I found that I easily memorized all the possible spell area effects.) The second rating will be "Usefulness;" how valuable it has actually been to me as a DM in play.
The things that I feel are all but "must haves": a megamat, D&D miniatures, and the Combat Pad. For DMing at a con, I'll add Dungeon Tiles and Flip Mats.