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.
David Kuijt has instructions on building terrain squares for DBA miniatures games.
Here’s a tutorial on making modular terrain from interlocking foam floor 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.
I’ve always wanted a good looking set of terrain tiles. Here’s a method for making terrain tiles out of flooring tiles. Its a thought I’ve had for while, although mine also involves 1 foot cork tiles stuck on top of them.