lets you easily setup IPSec VPN connections to another host or network over the Internet, while securing your data by encryption and authentication
pearPortVPN allows you to automatically establish a specific VPN connection whenever a wireless network of choice becomes available. Furthermore, it can also automatically disconnect any open connections before your Mac falls asleep and thus safe you from annoying error messages about timed out connections when your Mac wakes up again.
Shimo is an intuitive user interface for the Cisco VPN client. Features include: advanced statistics, uses Keychain for security, has automatic reconnect, and more.
Tunnelblick is a simple graphical user interface for OpenVPN 2.0 and higher. It is written in Cocoa and comes in a ready to use distribution with all necessary binaries and drivers.
(commercial) VPN Tracker is a VPN client which allows safe data transfer between your Mac and your company network. All unsecure Internet and Airport connections are encrypted on the fly by using high grade security algorithms offered by VPN Tracker.