Ion Thrusters?

There are those of us who hear this term and immediate have visions of particular fictional space craft (No matter how much we might try and deny it), but as it turns out the idea is actually plausible and doesn't sit only in the realm of science fiction. The idea is appealing not just because it sounds slick and futuristic, but it's also a much more efficient way to propel space craft than chemical reactions (about ten times as effective). The draw back is that such an engine can only exert the mildest of forces, and isn't capable of getting anything out of the atmosphere. However, once a ship is out there this can be used to gradually push the vessel to huge speeds.

NASA's Deep Space 1 spacecraft used just such a drive to thrust it's way to asteroid Braille and Comet Borrelly in 1999. There is no reason that such an efficient technology couldn't be used on mars bound craft and by now there must be drastic improvements on total thrust output. Another draw of such an engine is that it runs on gaseous neon and only requires electricity (which the suns light applies a never ending stream of) to make the thing operate.

Full article (also with minor notes on solar sails) : Skylights