Slashdot linked up this Gaurdian article detailing the latest efforts to build Leonardo DaVinci's "self-propelled transportation device." Previous attempts had failed due to a misinterpretation of the design, but by examining other related documents the correct drive mechanism was finally figured out.
What ended up being created was a roughly square vechicle about 5 feet to a side. It's propelled by springs and gears, which are tensioned by rolling the wheels backward (sounds like a car toy I had in grade school). It only turns to the right, and was even designed to be activated remotely. Despite its toy-like powerplant, the full scale version is so powerful the builders are afraid to test it.