I've started getting complaints from clients lately (on server 9), so I've been reading through the forum for more information on outages.
After reading this thread I thought I should mention a free tool that I use (not often enough lately, but now I am going to run it 24/7).
Karen's Power Tools - Net Monitor:
http://www.karenware.com/powertools/ptnetmon.asp
You run the monitor from your own PC.
You set the testing interval from seconds to days. (I have mine set to test every 5 minutes).
You can also put a "signature" (unique text) into a file on your server and choose to check that it is found in the file when you access it for the test.
Net Monitor keeps a log.
You can test multiple sites.
I use it in conjunction with Karen's Emailer II to send myself an email when a site goes down and when it comes back up.
http://www.karenware.com/powertools/ptmailer2.asp
This step is not necessary if you are at your computer as you can set the Net Monitor window to "Restore (take focus) and Beep" on failure.
Hope this helps,
Chuck