I liked how this website monitoring tool is positioned.
They have geographically dispersed monitoring locations which take the 'pulse' of your servers' health. They also have a "triple stage" error verification system and multi-level error correlated notification system.
This type of independent, off-site monitoring of the availability and performance of your web sites allow you to keep tabs on servers and network components, web applications and e-business transactions, web-based and e-mail systems.
This service ensures reliable early problem and error detection with a multi-stage verification process, followed by real-time alerts. Detailed reports and website diagnostic tools allow web system owners and operators to quickly locate and troubleshoot problems.
Note their feature set:
Network & Server Monitoring - monitors any type of server or network device as often as every minute with error detection and real-time multimedia alerts.
Website Monitoring - monitors performance and availability of web sites up to every minute simultaneously from a global monitoring network. Provides full-page downloads with real-time alerts generated if page errors or performance problems occur.
Transaction & Web Application Monitoring - monitors complex multi-step web transactions and the applications they use. Features simultaneous full-page downloads for every step from multiple cities around the World.
Email System Monitoring with Round-Trip - verifies that your incoming and outgoing email services are working from and end-to-end perspective.
Read more about WebSitePulse.