Xprintidle is a utility that queries the X server for the user’s idle time and prints it to stdout (in milliseconds). You could use it to check if anyone was sitting on your computer while you were away 😛 or even write scripts to perfom simple tasks like limiting your total idle time or send mails and notices.
# apt-get install xprintidle
while [ 1 ]; do if [ `xprintidle` -gt 120000 ]; then notify-send "You have been idle for 2 mins" break fi sleep 1 done
This script will send a notification when idle is more than 2mins and stop.
Be creative 😀