Tact-tiles, easily the best investment in gaming materials (not the PHB - they'll make the transition to 4E no problem). They work great for improve and you can explain as you draw a map of the kingdom, a unique trap, or something in section or 3D.
Along with the above is an array of different color dry erase markers to designate spell affect areas, secret doors, traps, initiative, etc.
Minis, of course, but beyond the WotC. Check the kids aisle sometimes and you can find some nifty stuff.
Some dungeon tiles that I use with poster putty to lock them in place on the Tact-tiles.
Jenga blocks; they are about 3" long, 1" wide, and 1/2" tall. Stack for 3D such as balconies, place under Tact-tiles for fighting on a hill side, or columns, walls, stairs, etc. Sometimes they can fall too - perhaps that character whose player with the shaky hand needs to make a Reflex when the player knocks one over.
Wide-format paper prints (up to 36x48) for some of the larger settings that the characters may need to defend.
11x17 prints for the smaller encounters, or just as something to get the players to illustrate where they are instead of 'just around the area'. It works well, as sometimes players have the habbit of getting lippy with the wrong shape-changed Rakshasa.