A nice tileset editor

It is not a famous one, but it is pretty good !

http://saqscrap.com/saq2d.aspx

The author give you source, in c#, and that's really important, as it is near impossible to find a universal tileset format.
In the same way, that's save the data in XML format, more easy to be processing after by your program, independantly of your langage.

Really a great job.