I was browsing Steam Greenlight and found this tileset making program.

http://autotilegen.com/

You just provide the bottom, top, side, and fill images and the program builds a tileset from that. You can also drag sliders to affect stuff. Rounded corners. What not.

It is 25 bucks, which isn't overly expensive. It might pop up on a Steam sale at some point though. Everything does. :P