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'