Kwout, the online screenshot service, has released an interesting Context Sensitive bookmarklet that automatically detects the relevant parts of a web-page at the time of screen capture itself so you don’t have to worry about cropping the useless portions later.
For example, note how the context sensitive screenshot of this Twitter status page automatically leaves out the Twitter logo and the background image.
Kwout’s context sensitive screenshot service is fairly limited at the moment, with support only for Twitter and Jaiku, but the developers say that will "expand its availability as fast as they can".
Related: How to Capture Web Page Screenshots