Check out www.smashingmagazine.com for 10 Designer’s Checkpoints To Be Aware Of article. It has a section about monitor.us:

monitor.us provides free real-time personalized Ajax dashboard interface, checks server performance and availability,
generates uptime and availability reports, tracks visitors, checks
system resources, and alerts its users in case abnormalities are
detected.
The service also provides tools for performance research and web analytics.
Thus you can use the service to benchmark your site performance versus
competitors effectiveness. So you can analyze web traffic tendencies
and observe the performance all in the same place.

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