Here is a new startup web service that monitors your website uptime and response time for serving we pages. The service is free. Here is a description from the MoniTwitter website :

"A global community couple of servers answering one simple question: What's your website doing? Receive answers on your phone, IM, or right here on the web!

• Receive notifications when your website goes down.
• Keep track of how long it takes your website to load.

MoniTwitter (Monitor + Twitter) was created by Jared Cosulich (CommunityWalk and Wedding Mapper) and Pivotal Labs as an innovative and useful way to leverage the communication network that Obvious Corp has built with Twitter."

As of 8 hours ago they claim to be monitoring 59 websites.