Q:

Where can I find free digital dungeon tiles?

I'm currently running a D&D 4e campaign on Roll20. I have been drawing the maps in Roll20, but it's a hassle and I'd like to switch to something pre-made. Most of the tiles on Roll20's site aren't free and I don't really like the ones for sale anyway. I also sometimes work in Masterplan, which really works best with tiles.
Where can I get tiles for free? Are there any alternatives to pre-made dungeon tiles for drawing dungeons online?

A:

The awesome dungeonmorphs are free in multiple formats! These are old-school in art style but made to match each other perfectly in terms of entrances and exits. I have the dice and the cards and love them. You get 90 of them in PDF, PNG, and dungeonographer (which is also free) formats.
http://www.dungeonmorphs.com/battlemats.shtml

A:

MapTool long predates Roll20, and while I won't recommend it as software (and it appears to no longer be actively developed), there's a large library of tiles (free and otherwise) that have been built up around it, which should all be compatible with Roll20.
Here's a few links to the more active resource sites, taken from the list here (and checked myself).

The Dundjinni forums
RPG Map Share
The User Creations section of the RPTools forum.

In addition to whole tiles, there's also pieces you can simply assemble into your own rooms - walls, floors, objects, etc.  Poke around and see what you can find. 

