There are a bunch of companies:
Steve Jackson Games has compiled all their cardboard heroes line of cardboard PCs, Monsters Dungeon tiles and Cavern Tiles in 3 spiral bound books for about $20 each. You just cut out and place.
Dwarven Forge is at the opposite end of the spectrum. If you're willing to shill out about $300 for the complete sets, they sell hefty boxes of hand painted resin dungeon tiles w/walls, special features and specialty rooms. These look spectacular, but the walls are kinda high--if figures fall over it's hard to pick them up (if you have big fingers like me).
Aintsy is a hard-to-contact company in England that makes moderately priced resin D&D and Sci Fi dungeon tiles.
There's some companies out there that make HIGH quality 3-D cardstock sets (I forgot their names) sold as PDF files. There's another company that sells molds for you to cast your own cheap resin tiles (I forgot that name too--sorry).
IMHO, your BEST BET is the new, reasonably priced 3-D Dungeon Tiles put out by Wiz Kids for Mage Knight Dungeons. They look great, are slightly larger than 25mm scale, clip together easily, come with walls and doors, and special traps and features.
There are 3-4 trap and dungeon dressing sets for about $15 each. The Tile sets are $30 each and come with: 36 floor tiles, 24 walls, 4 wooden doors and 4 iron doors. There are about 8 traps in a trap pack and 2 different dungeon dressing packs (fountains, chairs, tables, etc).
They look almost as good as the Dwarven Forge stuff (though more lightweight and less substantial). They only cost 1/3 the price!
