Seashore is an open source image editor for Cocoa (OS X). It is based on The Gimp (will read Gimp's .xcf files too), but since it is using Cocoa, it does not require X and it is much faster. Even though it is in version 0.1.7 I found it to be pretty stable.
The bottom-line: it's free, it works and it can only get better!
UPDATE: It will only open .xcf files...
