Seashore is an open source image editing program for Macintosh. It’s small and sleek. From the look of it, you’d think it was a very limited, simplistic program, and it certainly doesn’t pretend to have anything like the capabilities of Photoshop or the GIMP. But it has a surprising number of powerful abilities — painting in textures, multiple layers with many different blend modes (multiply, diffusion, blah blah), full alpha channel support, SVG import, and other things you wouldn’t expect out of a rinkydink paint program. It gets all that good stuff by leveraging the GIMP and the Mac’s native goodness — the guts are all GIMP code, the interface is all Aqua.
Unlike Photoshop, it’s Free Software.
Unlike the GIMP, it’s a native Cocoa application — no need for X11 to be installed, and it looks and acts like a proper Mac app should.
Unlike both, it’s super small — the entire app, installed, with optional extra SVG import added in, is about 8 megs on disk. The basic installer download is only 3.3 megs.
Also unlike both, it’s very new and probably hides some bugs. I’m gonna play with it a bit and report back. It looks like a very promising piece of Mac software.