What’s cool about Paint.NET is that it’s a free Open Source photo editing software. The other cool part is that it’s written in .NET… that makes it really cool. It currently is only available for Windows but Miguel de Icaza (of Evolution and Mono fame) did try porting it to mono and it was mildly successful. As Mono improves they hope eventually to be able to get it to run on Mono properly, which will mean that it will become available on Linux (and the other *nix that can run Mono) and Mac OSX.
Paint.NET is not a direct replacement for something like Adobe Photoshop or the Gimp but rather a photo manipulation application thats easy to use and does the stuff that most of us really want to do. This is going to be the photo editing software I recommend to all my friends for there Windows machines (replacing the Windows port of the Gimp) simply because it’s so much easier to use.