Editor: VPN Router has been discontinued as of 2009-03-24 partly due to poor donation turnout in comparison to download numbers. Feel free to continue using it and tweaking around in the script code, but don't ask about making it work on future Versions of Mac OS X. If you need support, please try a commercial software such as VPN Tracker.
Mac OS X comes with built-in Virtual Private Networking or VPN for short. However, configuration options are somewhat limited particularly when it comes to deciding which connections you wish to route through the VPN Tunnel.
Here's an example: Your local network is 192.168.111.X and wires your beloved Mac (192.168.111.10) and a printer (192.168.111.20). You'd like to establish a VPN tunnel to a remote network 192.168.222.X. Once the VPN is established, all traffic for 192.168.222.X is routed through the secure VPN tunnel whereas all other traffic is routed as usual. Unfortunately, there is a remote server 192.168.223.223 you should work on, but you don't reach unless you configure VPN to "send all traffic over VPN connection". Then again, if you do that, you won't be able to print on your local printer anymore.
Enter VPN Router. This little tool lets you define a list of remote IP addresses to which all traffic should be routed through the VPN tunnel. Technically speaking: It adds static routes to your routing table as soon as you establish a VPN connection.