G-Alarm is great and does what you're after. You can set it to make you do a maze and/or some math problems before you can turn it off. You can start with a demo from their site here:
http://ageye.de
If you decide to buy it, you can either buy it direct for $7.50 or so (they're European, so the exchange rate makes it an odd number), or it's slightly cheaper on MarketPlace.