xautolock

xautolock: start a program if the X session is idle for some time Xautolock monitors input devices under the X Window System, and launches a program of your choice if there is no activity after a user-configurable period of time. You can use this to automatically start up a screen locker if you have left your computer unattended for some period of time. The program launched need not be a screen locker such as xlock. From Debian 3.0r0 APT

* Linux/Unix/Computing Glossary Index