Especially of part of the screen, for quick uploading. Not comprehensive, just the top tools.
Mac OS X:
Skitch
Jing
Linux (Ubuntu/Gnome):
GIMP – File > Create > Screenshot (or File > Acquire > Screenshot)
scrot
import (ImageMagick)
Windows:
Jing
More lists:
How to Take a Screenshot in Linux (Ubuntu)
How to choose the right screenshot program
Taking a Screenshot
OSX Screenshot Showdown – Jing vs Skitch