Paint.NET is free, has lots of nice basic manipulation features, and is much, much easier to learn how to use than The Gimp/Photoshop, etc. There are a lot of things you can do with The Gimp that you cannot do with Paint.NET, but very few users would actually use any of those features.
As it requires the .NET framework, it is (for all reasonable intents and purposes) Windows only though.
04-03-10, 01:45 PM
There is also Picasa. (http://picasa.google.com/) I've used Paint.net and GIMP. Prefer GIMP, but it is a personal preference. I know where everything is and roughly what everything does.