URL: 
http://www.atstake.com/research/tools/index.html#network_utilities
 
Description:
Netcat has been dubbed the network swiss army knife. It is a simple Unix utility which reads and writes data across network connections, using TCP or UDP protocol.

URL: http://www.insecure.org/nmap
 
Description:
Nmap ("Network Mapper") is an open source utility for network exploration or security auditing.

URL: http://www.nessus.org/

Description:
The "Nessus" Project aims to provide to the internet community a free, powerful, up-to-date and easy to use remote security scanner.

URL: http://www.hping.org/

Description:
hping is a command-line oriented TCP/IP packet assembler/analyzer