virtualbox.org

Is it possible to make a VM automatically revert to a snapshot every night at a given time?

Yes. Just set up a cron job in the account that you run VBox from. Read up on the VBoxManage commands (controlvm and snapshot). Of course this will result in sudden death of any active sessions / processes and loss of information, so you might want to think through the implications first (like mapping a network share if you want to allow users the opportunity to maintain limited persistent data).