Dungeoneering dot Net has a nice selection of printable terrain tiles ostensibly for use with the Dungeons and Dragons miniatures game, but which would also work with any number of dungeon crawl games — of which there are many free ones available on this site.
terrain tiles
DBA Terrain Squares
David Kuijt has instructions on building terrain squares for DBA miniatures games.
Modular Gaming Board From Cork Tiles
I’ve been thinking for a few weeks now about using cork floor tiles to make terrain for a gaming table, and by chance I ran across these instructions on doing so.