Well, since we're going on and on, I do want to add that pingalink has clients like the United Nations, TDK, National Multiple MS Socitety, HP, and Pfizer keep me there. The price compared to the features can't be beat either...

You might also want to try PingALink. Pretty sophisticated and cheap by most standards. They helped me track down a bad DNS zone file I had and it saved my butt big time.