Well, there are two possibilities. You can go with GIMP if you'd like. Actually, if you're an avid Linux junkie, GIMP is what to go with. On my Windows platform, I prefer Paint.NET.
When you launch it, you'll notice that the interface looks terribly similar to Photoshop. All your toolbars and palettes are in the same place, they even give you a nice translucent look to them. Images can be managed easily from the preview on the top right, and you get most if not all of the control that you can get from Photoshop. Best part? Paint.NET is completely free to use.
My only drawback is that you need to have the clunky .NET Framework installed, which the setup does for you automatically. It's definitely faster than Adobe's framework.
OS: Windows Only