> comment on what they add.

Skitch adds annotations and sharing with almost the same simplicity as the built-in keystrokes. I typically use "⌘-Shift-4, Space" as well, when using the screens in my own work, but not when annotating or sharing.

For quick annotations, pointing things out, highlighting things, and instantly pasting someone a viewable image URL (with one click to capture, and one click to share), Skitch is tough to beat.