http://en.wikipedia.org/wiki/FLARM
Flarm is widely used in Switzerland.
Essentially it is a peer to peer collision avoidance system which is Open Source, it is simply a GPS attached to a small cheap computer and a low power radio.
The computer reads the GPS and broadcasts the a/c track & position on a public frequency.
The computer also listens out for radio messages from other aircraft and computes collision risk and displays any risk by way of a series of leds and buzzer.
The Flarm box has built in antennas, all it needs is a power supply, it is light and small enough to velcro on top of the panel.
It costs 600 swissies - approx £350