I don't want this to turn into a huge thread listing all current NASA missions, but I just want to put in a word for Kepler, which to me could be easily the most important NASA mission going on at the moment.
In 1994 we knew of zero planets outside the solar system. Ten years ago we had a handful. Now, thanks to the Kepler mission, we're on the verge of really understanding how common planetary systems are, how they form, the typical distribution of planet sizes and orbits, and how common Earth-like planets are likely to be throughout the universe. That's a big freaking deal.
In 1994 we knew of zero planets outside the solar system. Ten years ago we had a handful. Now, thanks to the Kepler mission, we're on the verge of really understanding how common planetary systems are, how they form, the typical distribution of planet sizes and orbits, and how common Earth-like planets are likely to be throughout the universe. That's a big freaking deal.