The Bias Drive is probably the most far-fetched form of space drive. Sometimes classified as a form of Pitch Drive, it involves the vehicle altering the properties of spacetime locally, usually the gravitational contant, G, to have a localized asymetric bias. By altering the gravitational constant directly in front of and behind the vehicle, a local gradient is created that propels the vehicle.
Besides the difficulty of altering the properties of spacetime, another problem with this form of space drive is the possibility of creating a singularity in the field gradient within the vehicle.
An illustration of the space-time distortions generated by a Bias Drive can be found at http://www.geosoc.org/schools/adult/docs/Image27.gif
Information on space drives: http://www.grc.nasa.gov/WWW/PAO/html/warp/ideachev.htm