I've used Paint.NET for years, but my needs in a photo editor are very, very basic.
I've heard that GIMP is a really good free alternative to Photoshop that has nearly professional level features, but you basically need to earn a Master's degree studying it to even have a cursory level of understanding of how to operate it.
http://www.youtube.com/watch?v=XiCtAUrZbUk
-- Defeating the Running Man of Ocarina of Time in a race since 01/17/2009. --