A follow-up on this old question.
If you've got ridiculous amounts of energy to play with, and not much reaction mass, a photon rocket is the way to go for moving your spaceship--it gives you the best possible power-to-thrust ratio.
Based on the comments section, this apparently needs some proof, so:
The relativistic energy equation is $E^2 = E_K^2 + E_M^2 = (pc)^2 + (m_0c^2)^2$ Note that, for a fixed fungible energy budget, the more energy we lock up in mass, the less there is to allocate to the kinetic energy/momentum term. Thus, the maximum amount of momentum per unit energy, and therefore the maximum thrust per unit power, is obtained when mass goes to zero, as we are left with the luxon energy-momentum relation, $E = pc$, or $p = E/c$, and the corresponding power-thrust relation, $P = Tc$. We only use rockets that throw mass out the back because we don't have a fungible energy budget--the mass-energy of, e.g., liquid hydrogen fuel isn't something that we can easily turn into photons. If it were, it would produce more thrust! So if you have a ton of energy and not much reaction mass, it is always a losing proposition to try turning that energy back into mass.
But, there's a problem: when all of the energy in your exhaust is kinetic energy rather than mass energy, it becomes much more noticeable! To merely suspend 1 kg of mass against gravity at the surface of the Earth, you have to provide 9.8 newtons of thrust--which corresponds to 9.8 newtons * 299,792,458 m/s = ~2.938 gigawatts of radiative energy. That's one frickin' powerful flashlight, just to suspend one kilogram. Actually accelerating upward, or suspending a ship weight many thousands of kilograms, requires proportionately more power, and exawatt flashlights tend to turn anything behind them into rapidly expanding plasma. If you want to launch from a planet... or just be in a solar system where you might at some point accidentally point your engine towards a planet... that's inconvenient, to say the least. Fortunately, it turns out photons aren't uniquely special--any luxon will do, as they all have the same energy-momentum relation! Including, for example, gravitons. So, if we have a magical means of producing high-power, collimated gravitational waves, they'll produce exactly the same amount of thrust, and interact with matter much less strongly, so we don't vaporize the spaceport when we take off!
Or... do we? Gravitational waves still do interact with matter a bit, so some energy will be transferred into the ground, and the rest of the Earth, if we fire a gravitational wave beam into it. Which raises the question: just how powerful can we make our graviton rocket before it becomes dangerous?