[Espy]

Here are the ones I've used:

Photoshop -- Great for coloring, contrast/balance changes, shading, and quick backgrounds. Not so good with tablet lineart.

SAI -- Really good for lineart, and has handy Flip and Rotate Canvas functions that make it easy to handle. It's also good at dumping colors (paintbucket tool), and you only have to go back and fill in occasional corners if you do it right. I'm still learning to shade, though. Can be...quite different from PS and Gimp.

Gimp -- I haven't dabbled much in Gimp, but then again, I didn't really know how to use art programs at the time. It wasn't as smooth as SAI, though...

Autodesk Sketchbook -- Not too many people use this one. It can be hard to handle at first, especially when, as a "sketching" program, it doesn't do too well in terms of solid colors. Neither does it have a good shading system, or dodge, burn, etc. but then again, I don't have the Pro version. It IS very nice for simple, quick doodles, though.