LiveQuartz, a free simple image editor for Mac OS X. Has standard tools and abilities like cropping, and can also apply many fancy CoreImage filters and effects. Lacks fine-grained selection control, such as for careful crops.

I'm stunned that I can't find a really good lightweight image editor for Mac OS X. There are dozens of screen capture tools, and a few decent sketching tools. But if I want to get a pixel-perfect crop of a screen capture for a production document, I need to open Photoshop. I realize that Lightweight Image Editor plus One More Feature That I Want turns into bloat very quickly, but even if the bloat ran its course, it still wouldn't be Photoshop, and would still open faster.

I'd be all about FlySketch for lightweight image editing if it were just a tad more bitmap friendly, like with zooming and cropping. Most of its features are vector-oriented, but a core feature is taking screen captures with its nifty see-through window, and there's not much you can do to the capture other than doodle on top of it. (Am I missing something?)