A (Mostly) Comprehensive List of Fantastic Terrain in 4E D&D
Here's a chart of interesting encounter terrains. Even if you don't play 4E, you might find some good inspiration.
Tile based mapping software makes creating floorplans fast and easy.
PyMapper is a great one because there are several tile sets available, some based on the official D&D tiles.
This tool creates appealing random dungeon maps. A couple of configurable options should give you thousands of hours of dungeon play.
Custom Monster Tiles
"I couldn't see buying actual miniatures, so I made these little bad boys instead. I used the Fiery Dragon token packs and bought some 1"x1" and 2"x2" acrylic squares from Litko."