Nessus is an excellent tool for the vulnerability identification and security posture assessment of enterprise networks. The application and its supporting plug ins are freely available for download. This is a Linux based application. Excellent installation and utilization documentation exists for this product across the internet.
Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics.
Internet Relay Chat is an excellent vehicle for the transfer of ideas and obtaining guidance on security related topics. The heavily moderated Hackerthreads IRC channel can be found at irc.hackerthreads.org in the channel #hackerthreads.
If you are providing penetration of wireless networks and infrastructure and have not read this book, then you should. You will not find a more comprehensive review of current and emerging technologies in the way of wireless hacking and penetration. If you’re a hacker or security auditor, this book will get you in. If you are a netadmin, sysadmin, consultant, or home user, it’ll keep everyone else out.