Linux open-source VPN packages

I needed to install an open-source VPN package for a Linux network, and had trouble finding a good listing of the options available or any comparisons between them. I did a little research and am posting the results here in case it helps anyone else.

I was looking specifically for Linux solutions, but most of the packages below have peen ported to *BSD and other Unix implementations.

(This research originally done in June 2002, with sporatic updates since then, most recently in Janurary 2010.)