Bussard RamJet; a large ship that gathers in hydrogen using huge magnetic fields. It is then super-heated using lasers. And the resulting plasma is fired out the back of the engine to move the ship along.
Not all of the galaxy has enough gas clouds to use this form of transport.
Dense gas concentraton: speed is up to three parsecs a week.
Medium gas concentraton: speed is up to two parsecs a week.
Light gas concentraton: use another means of transport
Beware ! As some of these ships advertise themselves as holiday excursions to many star systems, or a good all around means of transporting large numbers of people. These are actually generation ships built to look like Bussard RamJet ships and move no more than 1 light year per century ! Tau Zero effects do take place, but not in the early parts of the voyage, i.e. several centuries after leaving a solar system.