Web Based Network Diagnostic Utilities can prove extremely useful...
You are being SPAMMED? How do you find out where the junk is coming from? or How do you test the integrity of your firewall?
In these situations web based tools come into their own!
www.emailstuff.org/
Need to make DNS Queries? or Check Email Blacklists?
www.grc.com/
The Gibson Research Corporation applies itself to online security and privacy. The tools they provide enable you to test the security of your computer or network.
http://www.uwhois.com/
For Whois information about country-code (two-letter) top-level domains.
http://dig.menandmice.com/
Do a Men and Mice Dig if you need to query DNS information.