metasploit is another good one, although this one probably goes beyond pentesting...its more of a forensics tool.
Here's a good list of 5:
Here's someone's view of the top tools:
Both pages have good recommendations.
Note that you don't have 'real' pentesting tools to pentest. You can begin with Snort or commandline TCPDump or Wireshark (GUI sniff tool). Understanding things such as network traffic negotiation (three-way handshake and such) and protocols such as ARP and UDP will help a ton when looking at raw traffic with Wireshark or TCPDump.