Mac users
Screen sharing is already enabled between macs. For connecting to other computers, google "chicken of the VNC". You also have VNC server capabilities in Mac OS versions 10.4 and above (see link here to enable).

Linux users
http://lifehacker.com/software/how-to/set-up-vnc-on-ubuntu-in-four-steps...
or google "set up VNC $YOUR_OPERATING_SYSTEM"

Windows users
Try installing this:
http://www.tightvnc.com/
I believe it has VNC serving capabilities. At the least, it will allow you to connect to other computer serving VNC. Here is a link for what seems to be setup instructions for VNC serving capabilities with tightVNC, which should still apply even though outdated: http://www.yolinux.com/TUTORIALS/VNC.html
If it does not have serving capabilities (i.e. it doesn't act as a server):
Mac users can also connect to you via Remote Desktop Connection by installing the Mac Remote Desktop Connection client, if you enable Remote Desktop on your computer (google this).