cron-apt - Automatic update of packages using apt
This package contains a tool that is run by a cron job at regular intervals. By default it just updates the package list and download new packages without installing. You can instruct it to run anything that you can do with apt-get.
It also sends mail (configurable) to the system administrator on errors.
Observe that this tool is a security risk, so you should not set it to do more than necessary (automatic upgrade of all packages is NOT recommended).
More information on this package can be found on the Debian web site