What sort of propulsion would we require for interstellar travel? Further to this question I asked recently, lgritz makes a very astute observation about the massive fuel requirements to travel 36 lt yrs with known fuel technology today. So, if conventional rocket fuel is completely impractical, then what are the alternatives for interstellar travel? 
And how big would the spacecraft need to be, assuming that we could make a structurally sound spacecraft of the required size and then what would be the estimated fuel requirements to travel 36 lt yrs taking into account acceleration and deceleration?
 A: Rather than leaving a brief comment on this topic, let me just point at this wikipedia page which is very comprehensive:
http://en.wikipedia.org/wiki/Interstellar_travel
My own comments:
Once we learn to control fusion, that would be an attractive candidate for the engine. The nice thing is that there might be no need to convert the reactor's energy into something else (like electricity), then feed it to the engine. The reactor itself might be the engine. It would achieve fairly decent ejection speeds too.
In a more distant future, black hole engines look interesting. When talking about total mass conversion (into energy), most people think about antimatter; but a tiny black hole also converts all its mass into radiation, via the Hawking effect. If we learn to generate small black holes on an industrial scale, and if quantum gravity doesn't play some unexpected trick on us, those would make awesome engines - just feed them any random space junk and they keep going.
The only thing about micro black holes is that if you stop feeding them, they keep shrinking, and radiate even more furiously, and so shrink even further, and so on, until they detonate. Either keep feeding it, or eject it far far away.
A: Aneutronic fusion propulsion will be an important breakthrough for interstellar travel, because helium-3 and boron are easier to obtain in the universe than tritium, uranium-235, and antimatter. 
http://www.youtube.com/watch?v=sgAwyr5Udzw
http://www.youtube.com/watch?v=GSkxPghXTCg
A: something a bit out of the box would be to setup artificial black holes with the rotating axis aligned in the direction of desired travel, recreate artificially a relativistic jet emission, and it would be "as simple" as parasailing on the top of the ejected radiation. If you can survive the a-bit-warmer-than-tropical gamma radiation without melting (which is bound to soften as you accelerate anyways), you would accelerate to significant fractions of $c$ in almost no time.
The real problem would be to efficiently brake. I guess you could use a Bussard ramjet for that.
