Reader Kerry Fisher (no, not that Carrie Fisher), is concerned about the time her children spend in other worlds. She writes:
We struggle keeping our children in the real world. They would play online games such as World of Warcraft endlessly. There is a program named “Time Boss” for Windows that allows a parent to set time windows and limits on certain programs. Is there an equivalent program for the Mac?
You bet. Luma Code’s $30 Mac Minder can provide the service you seek. It allows you to set time limits for single applications or groups of applications (all games or browsers, for example).
Just choose a user account, pick an application or group of applications, and create either a simple schedule — one that limits the user to a specific period of time for the applications (an hour, for example) — or a custom schedule where you can muck with each day’s setting (an hour, M – F between 4:00 and 5:00 PM and then 90 minutes between 9:00 and 11:00 AM on weekends, for example).

The program requires that the administrator (a parent or teacher, for instance) enter a master password to create or edit schedules. Switching accounts won’t crack it — you still need to know that master password to work with schedules.
Version 2.5 adds the ability to set limits and options remotely. You can also log and graph use by day and user and track user logins and logouts.