Linux Google Groups

I am a fan and regular user of nearly all of these utilities.

A word to the wise: always seek out the original distributor (source code) to minimize the chance of receiving a "trojan-ed" version. It still may occur but it is simply one more step you can take to protect yourself. If available, use GPG/PGP signed packages or MD5 checksums to verify integrity. Try to run the latest version if you can. There have been attacks against libpcap aware applications. I prefer to run new software in a host-only VMWare network of three virtual machines. Two for client-server interaction and one for running tcpdump, ngrep and snort. This allows one to ensure the application is stable and doesn't try to "phone home".

The Linux Documentation Project

Here is a great free book on Linux/Unix use and administration