Night Night Mac App by Brice Milliorn
Night Night is a network activity monitoring tool that will put your Mac to sleep when your network activity drops below a user defined threshold of network speed & inactive time.
So the obvious benefit is you could start a late-night file download with an FTP, File Sharing, or any download client, knowing your Mac would sleep when the download was complete. Better yet, this program will work with any program on your Mac because it monitors network traffic from the operating system itself rather than just individual programs.
Developer Website: http://www.jbmjbm.com/nightnight.html