This was posted back in June to the sticky list of mapping software but I recently got pointed back to it by NathanaŽl Terrien's post on G+.
Pyromancers' Dungeon Painter is an on-line tool for building maps.
From Randy's post:
I played a bit with it and it seems a nice alternative to some of the older tile based mapping tools (Dungeon Crafter, Dungeon Forge, etc.) with some advanced tools like polygons and elliptical shapes as well.Free online mapping software.
Plenty of textures and objects,
layers and groups,
export to images and PDF,
hex and square grid,
simple file format, etc.
The wall builder seems a bit week, but functional.
It also supports layers, and can export as multi-page pdf files.
If anyone else has experience with this tool, I'd appreciate feedback here.