VPNs - Virtual Private Networks

VPN's are a way to establish a "virtual" network on top of some already-existing network. This virtual network often is encrypted and passes traffic only to and from some known entities that have joined the network. VPNs are often used to connect someone working at home over the public Internet to an internal company network.
If you are running a Linux masquerading firewall and need to pass MS PPTP (Microsoft's VPN point-to-point product) packets, there is a Linux kernel patch out to do just that. See: ip-masq-vpn.
There are several Linux VPN solutions available:

• vpnd. See the http://sunsite.dk/vpnd/.
• Free S/Wan, available at http://www.xs4all.nl/~freeswan/
• ssh can be used to construct a VPN. See the VPN mini-howto for more information.
• vps (virtual private server) at http://www.strongcrypto.com.
• yawipin at http://yavipin.sourceforge.net

See also the section on IPSEC for pointers and more information.