There is one idea that could work, but only within the solar system. That is the concept of solar sails. The scheme is that photons from the sun can reflect off of a 'sail' and thus propel the craft much as wind can propel a sail boat. I refer you to a nice wikipedia article on the subject.
I wish to point out a particularly relevant sentence in the article:
"The science of solar sails is well-proven, but the technology to manage large solar sails is still undeveloped."
LeeH
(published on 10/22/2007)
