nmap - Used for port scanning (used in conjunction with Wireshark it allows you to look inside the responses even more deeply)
netstat - allows (amongst other things to view TCP states) / "network status"
arpspoof (also Cane and Able / Eddycap ... "vs")
arp - allows you to view the "arp table" for a particular system
tcpdump (outputs in a format that can be ready by Wireshark)
Wireshark
Ping
Trace Route
Netstat (try netstat -l on Linux based systems to see all the applications and their associated ports that they are actively listening on)
Netcat (CLI name "nc") allows you to open and maintain connections to remote systems (e.g. port 80 on www.google.com and fire offHTTP GET requests). Supports either TCP or UDP based connections
Telnet - similar ti netcat except it doesn't maintain the connection and only supports TCP (and no UDP)