The Space Elevator is the most promising Space Transportation system on the drawing boards today, combining scalability, low cost, qualify of ride, and safety to deliver truly commercial-grade space access - practically comparable to a train ride to space.
Rocket-based space launch systems are inherently limited by the physics of rocket propulsion. More than 90% of the rocket's weight is propellant, and the rest is split between the weight of the fuel tank and the payload. It is very difficult (if not impossible) to make such a vehicle safe or low cost. A target cost of $1000 per kg is proving to be impossible to reach. In comparison, airliners charge us about $1 per pound, and train transportation is in cents per pound.
The Space Elevator is based on a thin vertical tether stretched from the ground to a mass far out in space, and electric vehicles (climbers) that drive up and down the tether. The rotation of the Earth keeps the tether taut and capable of supporting the climbers. The climbers travel at speeds comparable to a fast train, and carry no fuel on board - they are powered by a combination of sunlight and laser light projected from the ground. While the trip to space takes several days, climbers are launched once per day.
The first "baseline" design will use 20 ton climbers, but by making the tether thicker (which can be done using the Space Elevator itself) we can grow the Space Elevator to lift 100, or even 1000 tons at a time. In addition to launching payloads into orbit, the Space Elevator can also use its rotational motion to inject them into planetary transfer orbits - thus able to launch payloads to Mars, for example, once per day. Imagine the kind of infrastructure we can set up there, waiting for the first settlers to arrive... Looking back from the year 2100, the construction of the Space Elevator will be considered to mark the true beginning of the Space Age, much like the advent of the airplane or steamboat heralded the true commercial use of the air and sea.
See also our Space Elevator FAQ