Site Monitoring

Siteup - Freeware for windows (I assume there is something similar for Mac)

I've found this to be extremely handy for warning me when one of my websites goes offline.

We have about 15 websites on our books, so it is nice to know if one goes offline I am aware of it instantly, and at least prepared for a client phone call - but hopefully I've had enough time to find out what is going on and possibly rectify it.