[8088]

It's also very expensive. That is, if you can even find it, as it's already six years old.

[Tirant]

XnView is a VERY good free program. It has many useful tools that make it perfect for converting models and such. Photoshop no doubt has the best conversions and stuff, but it's a pain in the ass to use if you don't use it a lot. I've never tried paint.net, but I've heard it's good, as well as GIMP.