I've totally forgotten to show you this kickstarter by Manorhouse Workshop in collaboration with Maki Games:
Modular Underground Project
Being plastic and based on 10cm squares, it would be perfect for many a dungeon game, the upcoming Privateer Press Corvis Underground boardgame (though you'd need a fair few sets to remake the board with these sections) as far as fantasy goes. You could also use it for pulp games, or even sci-fi games (it reminds me an afwul lot of Hellgate London's sewer levels, for example, or some of the Metro 2033 stuff.
They've demonstrated its compatibility with several major other modular dungeon systems.
Here's what it looks like, once painted:
It currently seems to be flagging a bit, so please, have a look and if you like it spread the word!