Mokita - 11:22 pm on Nov 27, 2010 (gmt 0)
InternetSeer has a free service where their server requests the HEAD of your home page once per hour and emails you if it is unreachable. They keep testing every 5 or 10 mins after that and email you when it is working again.
They send a summary report once per week detailing any detected outages and also the connect time.
Obviously if an outage occurs and is rectified between the hourly checks, you will not know. But for a free service it is pretty useful.