After a couple of incidents recently where my (rubbish) Linksys router decided to silently remove my web forwarding rule I decided to try to put some kind of monitoring in place that my websites were actually working. A bit of Googling later and there seemed to be loads of these - I ended up with two of them!
http://basicstate.com/ allows a half hour check for a couple of websites and sends emails when there is a problem and daily with uptime statistics.
http://host-tracker.com/ is a similar proposition, though I have found it more fiddly to set up and not as reliable checking. Very nicely though it does let you have an image on your site with your verified site uptime:
It was 100% for ages but then I had a virtual machine go nuts for an afternoon.
Anyway, personally I found that a combination of the both of them works very nicely, and haven't found it necessary to upgrade to their paid service which checks more often and can send SMS messages. I have found it necessary to create copies of my website though with slightly different addresses, such as nolog.edjakeman.com, which dont log usage at all. I direct the checks at these, or I find that my web stats are skewed by a couple of obsessive fans checking the site 24 times a day each!