We along with the usual Nagios, we use Pingdom. It comes with lots of default checks.
For example it also reports how fast your website is or since they employ tests from different locations you get a nice graph how accessible your website was. To put some sense into it, add a reference check (e.g. google) and see how you perform.
Aside from HTTP etc. you can also check other services (mail, database, etc.). If they are not reachable from the outside, you can always create a script that outputs a standard "OK" and have Pingdom check on that, and report back if the output changed.
I should add that Pingdom is not a free service. But we've been using them for 10 months now and they haven't troubled us. :)