InternetSeer - FREE Website Monitoring Service

Use InternetSeer's "free" package to monitor your Web site's uptime (or downtime!) on an hourly basis. The service will probe your site and check for any problems that might prevent visitors from viewing your pages. If a problem is found (such as a connection refused error), they will automatically send you an e-mail and monitor progress until your site is back online. They also offer commercial versions of the service, which provide more frequent monitoring.