CheckYourNetwork.net - Free Online Network Checking and Testing Tools

ShowMyIP.com - IP Checking
    showmyip.com is the Internet's premier IP address web site. Here is where you easily find your unique Internet Protocol address and associated host (DNS or Domain Name System) name; Internet Service Provider (ISP) name; Organization that owns your IP Address; browser type, version and exposed headers, and more....
SamSpade.org - Various Network Checking Tools.
    A Great Place to Start. whois, traceroute, finger, ping, DNS query, NS lookup, host lookup, reverse dns digger.

Be Sure to Check out Clustered Networks Untangle Security Appliances

The Untangle Gateway, the world's first commercial-grade open source solution for blocking spam, spyware, viruses, adware and unwanted content on the network, provides a free and better alternative to costly, inflexible proprietary appliances. Untangle's applications are currently being used in tens of thousands of businesses in a variety of industries, including financial services, real estate, education, bio-science, and professional services.