Another free program that works well is Paint.NET. It isn't as full-featured as GIMP, but the interface is a bit more standard, and it has certain tools which GIMP lacks, such as ones for drawing basic shapes (in GIMP you can do this too, but it requires more work). The main thing it lacks is the ability to edit the pallete. You can, however, save it as a 256 colour bitmap and then edit the pallete with an external tool. Overall I like it more than GIMP, at least when using Windows. It's a lot faster too.