Hamachi runs on Mac OS, Windows, and Linux. With Hamachi, you can create a private virtual network that allows you to communicate amongst the various machines that have the Hamachi client installed. Unlike the Windows version, the Mac version is currently command line only, but can be fairly easily installed by following these instructions from the Hamachi forums. Note that you'll also have to install the OS X TUN/TAP driver.
[robg adds: I haven't tested this one. If you are going to do so, I would strongly suggest you check with your IT department first -- many companies, such as one of my previous employers, have very harsh policies when it comes to circumventing firewalls.]

