I recently downloaded Cool Timer (free) to help control the time my family spends on the computer. It's all to easy to sit down to a task and end up getting distracted with news, email, games or whatever and find that the original task took a whole lot longer and anticipated to complete.
Cool Timer is a simple, straight-forward timer that even my kids can use. I have them set it before they start on the computer. It is understood that when the "fog horn" (my son's favorite alarm sound) goes off, time's up. One less thing I have to keep track of.