Apple-Shift-3 captures the entire screen.
Apple-Shift-4 lets you select an area to capture.
Apple-Shift-4 then Space captures a window that you select.
I don't personally use GrabUp or Skitch, so I can't 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.