A NASA contractor has built a test engine that runs on methane. That image is from a movie of the test firing, and yes, you want to see it. Make sure the sound is turned up. So so cool. Methane is a pretty good fuel. For one thing, it doesn't have to be kept as cold as other fuels, making it easier to make and use. You don't need such thick tanks for it, which saves weight, and it's a safe substance (a spill wouldn't be all that toxic). The linked article about this is pretty informative, but one thing it doesn't have is the comparative thrust of methane versus, say, what's being used now on expendable rockets. If the bang isn't worth the saving in bucks then it hardly matters. This particular engine had a 7500 pound thrust, which is nice, but compare that to the 100,000 pound thrust of the GEM-40 (used on Delta II rockets) -- and bearing in mind that several GEM-40s may be strapped on to a rocket -- and you can see there's a ways to go yet. Still, it's early in the game, and this was a test of a small version. I'm sure they'll get bigger! And when they do get bigger, and can be used for solar system exploration, there's another advantage that kicks in: methane can be found all throughout the solar system (or made easily from available materials). That means you don't have to carry all your fuel around with you! If you go to Mars, or Saturn's moon Titan, you can bring enough fuel to get there, then make the rest from the planet you're sitting on (and please, no astronaut diaper or related jokes!). That saving in weight is HUGE, and a very big plus. And also, really just how frackin' cool is that video? Wow.