Some key concepts and work here or which have been available for some time:
Magnetic systems that can utilize the solar wind or use a magnetic plasma.
Advantages: reasonable power levels, does not require megascale engineering
Magbeam, Magbeam reference and plasma magnet
Magbeam can use 3,000t of current batteries or 150t of fuel and 1 ton of MNT fuel cell @ 4000 ISP to accelerate 10 tons of payload to 20 km/s or 72000 km/s
Plasma magnet able to capture solar wind could accelerate 300-800km/s.
We are just starting to make them now. They have slow acceleration.
Nanotechnology allows very fast acceleration.
A close pass slingshot around the Sun (3 solar radii) allows acceleration to up to 13% the speed of light. Such a close pass allows for someone to just make the solar sail and not have to make a giant laser and lens to accelerate the solar sail to fantastic speed.
Current work on ion engines:
The European Space Agency and the Australian National University have successfully tested a new design of spacecraft ion engine that dramatically improves performance over present thrusters and marks a major step forward in space propulsion capability. The new experimental engine, called the Dual-Stage 4-Grid (DS4G) ion thruster, was designed and built under a contract with ESA in the extremely short time of four months by a dedicated team at the Australian National University. The test model achieved voltage differences as high as 30kV and produced an ion exhaust plume that traveled at 210,000 m/s, over four times faster than state-of-the-art ion engine designs achieve.
A hypersonic skyhook is an orbiting tether. It is a pre-cursor to a space elevator. It could reduce launch costs by 4-10 times. A hypersonic plane that can go mach 10-15 would rendezvous with the skyhook and get payload to orbit.
hypersonic skyhook info
detailed hypersonic skyhook study
A lunar tether system we can make and launch now if the funding and motivation were there
another study of a lunar space elevator
Announcements of Japan has lunar ambitions
and the plans of China, India, Europe and the United States.
Antimatter storage is starting to happen now. There is an NIAC study on collecting antimatter that is in the magnetic fields of planets. There are also NIAC studies on efficient systems for using antimatter for propulsion.
Sufficiently strong magnets would allow for a ground-launched system that pushed against the magnetic field of the earth. Room temperature superconductors would likely allow this to happen.
Past system that is technologically feasible is project Orion.
Challenges of Molecular Nanotechnology for Space Exploration, Thomas L. McKendree, Robert A. Freitas Jr., Al Globus, M. Creon Levit, C. David Sherrill , Mo Li and Ralph C. Merkle 2005.
One potential of molecular nanotechnology is to fabricate structural elements for space systems of very high strength-to-weight, including diamond compressive members, nanotube-based tension members, and composite structures of nanotube fibers in a diamond matrix. Diamond has a σ of 5.0 x 1010 Pa with a δ of 3510 kg/m3, and thus a strength to density ratio ~70 times better than Titanium. Nanotubes have a σ of 4.5 x 1010 Pa5 with a δ of 1300 kg/m3, and thus a strength-to-density ratio ~2.4 times better than diamond, but only in tension.
Such materials, if used to reduce parasitic rocket mass, can reduce rocket dry masses by ~98% and thereby triple rocket payloads to Earth orbit. Depending on the cost model, this can improve launch costs by a factor somewhere between 3 and 235
Another potential of molecular nanotechnology is to provide mechanical devices with part sizes down to molecular components. Excluding systems which use such devices to fabricate products by rearranging molecular structures, other application to space exploration has been identified.
Designs in Ref. 4 use tiny motors to individually steer reflective solar concentrators 0.1 mm in diameter. Their small size allows them to hold optic tolerance while presenting 3 x 10-4 kg/m2 of mass per unit area to the Sun, and this raises the specific power to 739 kWe/kg, available for a further factor of 70 improvement in solar-electric ion engines and other purposes. At this level interplanetary trips can take weeks with reaction mass a minority of initial vehicle mass
McKendree, T. L., “A Technical and Operational Assessment of Molecular Nanotechnology for Space Operations,” Ph.D. Dissertation, Industrial and Systems Engineering Dept., Univ. S. Cal., Los Angles, CA, 2001.