A sky ramp is a rail road on the side of a mountain, pointing east towards the sky. A rocket sled runs along it which acts as a reusable first stage for a rocket.
See also the Mass driver entry for an alternative propulsion method.
A rocket sled launch on a mountain side, will fire a space craft at the sky. While the ship is on the sled it fires its rockets and will rise as its cradle drops out from below it and decelerates. This gives the space craft a significant boost to its speed, start its journey already above a significant portion of the atmosphere, and being farther from the planet's centre, loses less energy to gravity drag. (On Earth, there is an additional, significant boost, in that it can start with vacuum optimized engines rather than sea level engines).
After the space craft has left, the cradle decelerates, rolls back down the rail way to its staging area, refuels, and is quickly ready to launch the next space craft. Effectively it is a 100% reusable first stage with a turn around time measured in minutes or hours.
To maximize the advantage of the planet's rotation, a long gentle slope on the equator and pointing east would be idea.
Possible Martian Location:
Mars has a marvellous location for a sky ramp: Pavonus Mons on the Tharsis Bulge. It is barely north of the Equator, and rises to 14 km above the datum (the average Mars surface level). Thus the top of the ramp would be above 90% of the Martian atmosphere.
Note that this location is also ideal for a space elevator. A sky ramp could be built cheaply, and later be supplemented with a far more expensive Space elevator. Note that the space elevator must be exactly on the equator, so it would be slightly south of the volcano's peak.
// Website giving details of sky ramps on Earth. On Mars, these would allow a single stage to orbit with a wonderful mass ratio.
// Wikipedia entry on them.