I somewhat recently came across AutoTileGen by Pixelatto—a tool for rapidly creating "blob pattern" tilesets from just three input images. It seems like a pretty good tool, at least for a quick rough-in of tiles. It just has one major flaw—the tilesheet that it outputs is all mangled:
So I created a simple tool in Python to covert the above output in something a little more friendly:
You can get it (and the source) on GitHub.