Droplr [1] is excellent, and free if you don't need the uploaded stuff to be saved for ever.

It does both screenshots or screencasts (MP4 and GIF, no audio), supports the same keyboard shortcuts as OS X (region, window only, etc.), has Skitch-like markup tools for drawing onto screenshots, and has a very nice little dropdown menu from the system menu bar.

Droplr can even do screengrabs of entire web pages (as long as they are public, since it's rendered server-side). Example: http://d.pr/i/14cdd.

[1] https://droplr.com

I recently purchased Dropshare[0] when it was on 'special' for $0.99 on the App store. This is more targeted at being able to send the screenshots to your own server (via scp) but has screenshot and video recording support.

Though, I imagine you could easily achieve the basic functionality by writing a quick script to run the cmd line 'screencapture' and tying it to a hot key combo.