I'm using Gimp in Linux and I love it. It will probably take some time to get used to it when you're used to Photoshop. But there's also a Gimp-Version that looks like Photoshop (don't remember the name though).

Potoshop has a few more functions but you can install a lot of plugins in gimp and it's also very easy (at least in Ubuntu).

There's also a Gimp-Version for Windows if you want to try it out before switching OS completely.