LiveQuartz is a simple and powerful photo editor based on non-destructive layers and filters. The layers are at the left and the filters are at the right of the window.
To edit a photo, just drag it into the window (or layers list) and it will be added as a new layer. LiveQuartz handles multi-touch trackpads. You can watch a screencast of LiveQuartz in the help menu or on this page, where the screenshots are located on the right
LiveQuartz Lite can export or print one image per day (export, copy, drag-and-drop, print, etc.). You can remove this limitation by upgrading to the unlimited version and fully enjoy the application. A dialog will appear in the app when you hit the limit.