Philip’s post about More OS X Leopard Tips & Tricks is quite dated, but basically, all the valuable tips there are still valid.
Among them also the tip how to get rid of the (usually annoying) drop shadows that Mac OS X adds to screenshots (enter the commands in a terminal window):
defaults write com.apple.screencapture disable-shadow -bool true
Adding shadows again:
defaults delete com.apple.screencapture disable-shadow