At a Distance is free, and available to download for Windows and Mac.
At a Distance can be somewhat difficult to set up, but the basic principle for both Windows and Mac OSX is: (1) host a server on left brain, (2) make sure it's not firewalled, (3) find out the left brain's IP address and (4) connect as a client from right brain.
While it is technically possible to run the game online, the game isn't designed to be played that way. It is important that you can always see the other player's screen and communicate with them.
Step by step guide:
(1) Ideally, the two machines should be side by side, and sharing a network. You can also connect them directly with an ethernet cable.
(2) If you want to use Xbox 360 controllers, connect one to each machine before starting the game.
(3) The recommended settings to run the game are "Good", width 1280, and whatever height matches your screen aspect ratio.
(4) Start the game on the left machine and select "Start Left Brain". After it has loaded, click "Start Server" in the top left corner of the window. On Windows, the first time you do this, you'll probably get a firewall warning in the background. Alt-Tab to it and allow it access.
(5) Find out the IP address of the "Left Brain" computer:
ON WINDOWS: Open a command prompt by typing "cmd" in the start run menu, type "ipconfig /all", and note the IPv4 address. It's likely to be something like 192.168.1.x.
ON OSX: Follow this guide to find out your IP address. It's likely to be something like 192.168.1.x.
(6) Once you have the IP, run the game on the other machine and select "Start Right Brain". After it has loaded, insert the IP address of the Left Brain machine and join as a client. If it works, you'll get a chat box! Type something on both machines (press Enter to send) to make sure they're connected.
(7) Last step - if you're using a controller, you may need to adjust the controller sensitivity. Use the + and - keys to adjust it until you can look around with the right analogue stick comfortably. (You may need to press the - key several times if you're using a very fast computer.) You'll need to do this on both PCs.
(8) Finally, press F5 on both machines to hide the networking GUI, and start playing!
See the FAQ page for troubleshooting.