I've been looking around for a freeware or Open Source Mac OS X lightweight graphics editor for a while now. Paint.net has been serving that application space well for me when I use Windows but I didn't know of one for my Mac until now:

Seashore

...is an Open Source Mac OS X image editor. It doesn't have all the features of Paint.net but it has enough to take care of those quick little tasks like cropping and resizing an image for a blog post. The latest version (0.1.9) became available on April 5.