I've always wanted a tool that can automatically suspend my system automatically once all the jobs (mencoder scripts, updatedb, etc.) are finished. Then I could start tasks, leave my system alone and trust that it would take care of going into standby itself. Most software only checks for keyboard and X input, which exactly doesn't help for my scenario. But now there's "powernap".
It's included in normal Ubuntu repositories and in its default configuration it suspends your system if no activity is detected for 5 minutes. What's still confusing me is that it by default checks for activity of initd. This does not seem to make much sense and I think a long list of typical software would be a much better alternative. Also I wonder whether it would also detect mouse and keyboard inputs, which might be nice and useful. But it's a good start in any case and you can set it up to run for your purposes.