I've been using my own system for about a year now to monitor my sites and keep an eye on whois information, expiry dates, changes, etc. I've now put a site around it and opened it up to the public as well as adding more monitoring servers. Basically it can check your websites are running (not just existing but actually functioning). It also keeps track of your domains, registrars, nameservers, expiry dates, and whois data and automatically emails you (up to 3 addresses) if it detects any changes. These are checked every few minutes. The site is at AlertDomains.com - there is a free account option there to try it out. Any questions please email me.