Beacon
A basic beacon is a device that transmits module effects to nearby machines. The effects are not transmitted with perfect efficiency. The effect on each machine is decreased compared to putting a module directly into the machine. However, a basic beacon allows you to transmit the effect of a module to multiple machines and the effect of all beacons around is added (it might make sense to use more beacons in a dense area). Additionally, beacons can add qualities beyond the normal effective module cap of an item, such as boosting a Pumpjack's output at peak oil to .25 oil per cycle where, alone, it could only reach .2 yield.
Currently, only speed and effectivity modules can be used in the basic beacon, and productivity modules cannot.
The Beacon is a building that spreads the effects of inserted Modules to nearby buildings touching the effect-radius, but only half the benefits. This effect is called "distribution efficiency".
Multiple beacons can affect a single building, even one that is already full of modules.
They affect only buildings with module slots (laser turret doesn't benefit). Beacons themselves don't benefit from the modules inserted, so their energy cost can't be reduced.
They can't be loaded with productivity modules.
Usage
Beacons are extremely specialized since you need to focus your buildings around a beacon layout to be worth placing.
Their need for power makes them only useable with serious planning!
Examples
See this forum thread.