TileStudio

Blurb: Complete development utility for graphics of tile based games. Includes a bitmap editor for creating tiles and sprites and a map editor for designing level maps. Output is completely programmable, generate header/source files for use with any project.

http://sourceforge.net/projects/tilestudio/

*EDIT*

Has support for Blitz as standard :)

Should mention this one here as well, also has support for Blitz:
http://www.tilemap.co.uk/mappy.php

I have worked with both and can only say, choose Mappy.

Heres a little tutorial I've put together.

Link.