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 is a free update for registered users of version 2.x.