If you can set up a Linux box and have your own 24hr connection, there's also a program called "Webmin". One of the modules is called "System and Server Status". This can be set up to monitor whatever you want as often as you want... and email whenever something is wrong.
Only downside is that YOUR connection might have a glitch every now and again, so a blip here and there isn't conclusive. If you monitor 2 or 3 different servers/hosts and only one is repeatedly down... then there's a good chance that the host going MIA periodically.
http://www.webmin.com/
('System and Server Status' is one of the standard modules)