Cross-platform parental control: http://logmykidsoff.com lets parents limit the time their kids spend in front of computer, games console and TV screens.
As a parent, you sign in to logmykidsoff.com and then download the "monitor app" to the computer your kids use. During installation, you "link" your kids to your logmykidsoff.com account. When your kids log in, they now have a time allowance. When they have 5 minutes remaining, they see a visual and audible warning. At 1 minute to go, they see a second warning. When their time runs out, the screen goes black and they get logged out. If they log in the next day, their time remaining gets reset to the allowance.
You can modify the time remaining at any time; our experience is that this happens frequently, so that computer time can be co-ordinated with other family activities.
The kids usually use language like "the computer logged me off" which shows how LogMyKidsOff can really help to reduce family conflict around computer time.