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.
> It needs zero propellant, and zero energy input <.

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