Wireless network
If you don't need extreme speed, you could use one of those OEM RF Transciever modules, like the ones at Linx and Laipac. Of course, you have to figure out the networking protocols yourself, and you'll have to use some kind of encoding for reliability, like Manchester. If money isn't too important, MaxStream makes a nice Spread Spectrum transciever, with all the networking and encodinig done for you. All you do is connect them to a serial port. They are about $100, but go down to $60 in quantity.