Pinta -- Free Paint Program for Linux, MacOS, and Windows
Paint.net is a nice program. It is far more powerful than programs like Windows Paint, it is easy for the average person to use (unlike programs like Photoshop), and it is free. Unfortunately, it is a Windows only program. If you work on different computers with different operating systems, it can be a pain to learn different paint programs -- especially if you only use them once in a while to create an icon or retouch an important photograph.
Pinta to the rescue. Pinta is a cross-platform freeware paint program running on Windows, MacOS, and Linux. While it is not as powerful as some, it is far more powerful than the free paint programs that come with most operating systems. It is fairly easy to use, but has a good number of powerful features, including unlimited layers, full history undo (making it is to backup when you find that your multi-step "improvement" wasn't quite the improvement you expected it to be), a good number of adjustments and effects, the the basic drawing tools you expect from any good paint program. While it will never replace Photoshop, Pinta is an excellent paint program to consider if you would like to use the same program on your Windows box at home and your Mac or Linux box at work.
The only drawbacks are the extra requirements for some system. Mono for OS X needs to be installed on a Mac and GTK# needs to be installed on a Windows system (along with .Net 4, but most modern Windows setups already have it). Links to both are available on the Pinta download page.
Operating System: Linux, Mac OS X, Windows
License: Freeware (MIT X11 License)
Version Tested: 1.3
Web Site: http://pinta-project.com/