I've just released TaskMonifier -- a simple (FREE!) application that will
monitor the processor, network or disk usage and not
ify you when this drops below a specified level.
http://taskmonifier.ideasasylum.com/
It is ideal for detecting when a huge download has finished, when your hard disk image has been created or when those photos have been imported. I wrote this utility as part of my PhD to inform me when my calculations had finished. Once the level drops below the threshold, TaskMonifier will perform your selected action (play a sound, hibernate/shutdown, send an email or even an SMS to a mobile phone).
Let me know if you've found it useful too.