Chronic comes with several built-in constraints, and provides an open, plug-in architecture for writing and using new constraints.

Still in beta, Chronic is already a very useful tool for scheduling tasks based on various measures of system inactivity. The chronicd manpage details usage and describes all available constraints. Chronic's design is still not finalized, so if you want it to do something differently please let me know. All comments and feedback are welcome and can be sent directly to the author.