Flying Meat has updated Acorn (http://flyingmeat.com/acorn/), a layer-based bitmap image editor. A free update for registered users, to version 2.5.1. The new version is a maintenance release with some bug fixes.
With Acorn, you can take screenshots and edit them immediately. You can chain together image filters to create effects. You can make new images and layers using your built-in iSight. Acorn offers layer-based image editing and image and canvas resizing, just by changing the size of your window. The software works with drawing tables. Plus, you can write plug-ins using the Python scripting language, as well as in Objective-C.
Acorn is Universal Binary so runs natively on both PowerPC and Intel Macs. It requires Mac OS X 10.4.9 or higher. It costs US$49.95, but version 2.5.1 is a free update for registered users of version 2.x.