This may be the single most useful tool you'll find in the Internet: A wholly customizable Dungeon Generator. Just enter general dimension, denseness, even a % chance for secret or concealed doors, hit ENTER and -zoom- you're there.

Here's the link – and here's a "random example":

dunni1.jpg

Notice the nice touch of the graphics? Like in the good ol' "D&D 1st edition" days. Oh, and did I mention that the rooms are already randomly filled with monsters and include difficulty numbers for skill checks and all?

P.S.: The website also features a Random Town Generator, a Random Treasure Generator (actually they are two: one by item type, one by encounter) and a Random NPC Generator in 3.0 and 3.5.