Sentinella

Sentinella is a free desktop application that monitors your system activity and, when a condition is met, takes the action that you've chosen.

Thanks to libsysactivity it can measure the activity of your processors, memory, network interfaces (and more) and whenever one of these conditions reaches some limit it will automatically execute a chosen action, such as shutdown/reboot/hibernate your computer, kill a process, trigger an alarm or execute any command.