I could see http://downforeveryoneorjustme.com/ being useful from time to time, checking connectivity (um ... yes, you can check other domains, not just Google.com). The first time I tried it, it was down. Which promised to become a bit recursive. But now it's up. Hopefully.

In a similar vein, a web-based email checker http://hexillion.com/asp/samples/ValidateEmail.asp (but read the fine print before getting too excited about any supposed address validity).

And a web-based traceroute has been mentioned in its own topic - http://forum.spamcop.net/forums/index.php?showtopic=8216