site24×7.com is a web site monitoring service from Zoho that provides a more complete feature set than other free and low cost services I’ve looked at.  (It supports response time and availability graphs and reports, multiple http: and https: urls, test intervals as short as 5 minutes, monitoring for the presences and absence of keywords, and checking for web site defacement.)

The service currently totally free in beta, final pricing is not yet disclosed.