Tether

Possibly the simplest and most elegant approach is the method of momentum exchange by tethers, from Tethers Unlimited.

This could be built fairly easily, possibly cheaper than building a mass driver on the Moon.

http://www.tethers.com/papers/CislunarAIAAPaper.pdf

It has an advantage over mass driver in that it can be used to soft land on the Moon as well as depart from the Moon.
If the energy imparted by the cargo is balanced, the tether would require little if any energy input and minimal propellant usage

External link: http://www.tethers.com