It's Your Mac - Control who uses it
Downtime - Parental Control for Macs
Downtime lets you set rules to control how much time different users can spend on your Macintosh. Downtime works with the multi-user features of Mac OS X, Jaguar, Panther, and Tiger.
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When a user's time on the computer is over, they are warned to save changes. Then they are logged out and can't log back in again till the time you specify.
Read the online manual to see a description of all features and screenshots.
