A listing of some great Network Security tools (many are free, like Wireshark) that work with different platforms is at: http://sectools.org/. The list is from 2006, but most of the tools are still available. The site includes some nice summaries of each tool, and which platform they are available to work with (Windows, Linux, Mac, etc...)
The list is also quite humbling in a way because it includes some tools that could by used by end-users against your network. But it's good to know what tools are out there