HI there,

I've recently set up a site to keep DNN sites alive / Monitor response times.

http://www.pingalive.com

It's totally free, hits your site(s) once every 2 minutes, logs the response times of each hit and produces detailed graphs of the results.
Oh, and it alerts you when one of your sites experiences problems.

It's a spare time project of mine, so if you have any suggestions, please let me know via the site.

rgds

David