On Windows, try out Paint.NET. It's free, easy, and pretty powerful. You won't be able to do everything photoshop can but it gets the job done for most things.
On Mac, try out Acorn or Pixelmator. They're about 50-60 dollars but they're great programs and have free trials.
Another thing to try out is The GIMP. It's free, open source, powerful, and not very user friendly. You can use it on Windows or Mac.