scrot
Scrot is definitely a cute but powerful screenshot tool.
Fullscreen shot
scrot a.png
Manually pick a window
scrot -s a.png
Countdown (specified with -c, and delay with -d, usually used together)
scrot -cd 5 a.png
With window border
scrot -b a.png
With thumbnail
scrot -t a.png
Execute a command on the file ($f represents the filename)
scrot a.png -e 'echo $f'