This is a handy little utility provided with Mac OS X. It will allow you to:
Check for open TCP ports
Check the status of/investigate your network connection
Look up user/domain information
Test whether you can contact another computer
Test your DNS server
Trace the paths of your network traffic
View AppleTalk routing tables and statistics
View network routing tables and statistics

It does this using one of Info, Netstat, AppleTalk, Ping, Lookup, Traceroute, Whois, Finger and/or PortScan.

If you're looking for the app that provides a variety of computer network information, this is it and it's already included with OS X.