I've got the following thing that I use EVERY NIGHT (hopefully can upload a picture):
1. Battlemat (duh!)
2. model railroad trees and shrubs (CHEAP FOLIAGE)
3. A few sticks and rocks from the yard (and some rocks with fossils that I've collected from dinosaur ridge nearby..because God made fossils to be played with, not stared at in a museum). I keep flat sandstone rocks around because it's best if you can set miniatures on them. The archers always want to be up on them playing sniper.
4. Black dominoes for corridor walls rather than markers
5. Cardboard Doors: now these I got from some old D&D products (kidnapping of princess arlena and revenge of rusak)
6. Cardboard buildings from various products.
Other stuff:
* Modelling clay
* Dungeon Tiles (Wotc just put out some nice new ones). These can be anything from trapdoors to tables, chairs and candelabra. They are cheaper than the plastic ones and still work well.
* Hard foam painted up to look like cliffs (get some ideas from warhammer)..this was kind of fun ot make. I just pulled some crappy ugly leftover paint out of the garage, painted the hard foam and sprinkled some very fine sand and dirt on it.
* Trinkets, clear plastic, and odds and ends. You'd be surprised what you can use to make tiny props out of. They don't have to be perfect, just representational.
* CASTLE WALLS: I highly recommend that you purchase the old product "Castles" for AD&D. It has a crapload of stuf fyou can use. If there's one thing that you need, it's castle walls. THere are always encoutners where goblins are shooting off of crap or things for the rogue to climb up to.
3D has made our game a LOT more interesting beyond the primitive old days of 2d markers on a battlemat..or worse..staring at 0-D on a computer screen...gaaak.
Oh, one more thing: we have picked up a few ideas from the LARPers. We keep some traditional design stoneware/mugs/jacks to drink out of and for theme. Everything helps:
http://www.tokheim-stoneware.com/
jh