Paint .NET is a great little app. It's entirely open-source, and contrary to an above comment, none of the head developers work for Microsoft. They're all students at WSU.
They do have a "Microsoft Mentor," Rick Brewster, who helps them out and has done a great job helping to spread the word about Paint .NET, and was one of the original designers before he went to work for Microsoft.
I've been following the project, as well as Rick's blog, for a while. You can check it out here if you're interested:
http://blogs.msdn.com/rickbrew/ Also, has anyone tried running Paint .NET on Linux (with Mono)?