[kilonox]

Gimp is great freeware that is cross compatible. Though it takes some time to learn. I would also recommend paint.NET as it is not quite as flash, but lacks some features Gimp has.

 

[kilonox]

Photoshop CS5 is obviously one of the higher recommended, but again, takes while to master like Gimp. Its power is almost unrivaled, but it carries a hefty price tag: $700