We already have a bunch of robots exploring various aspects of Mars, but even though we're not running out of things that bots can do in space, it would be nice to get astronauts walking on another planet someday. We could go back to the moon, but Mars is a more ambitious and noteworthy goal. Getting to Mars safely is a challenge because it'll take several weeks (at least) to get there, and no one has ever tested radiation shielding on such a long trip in open space. Still, it should be possible if we're committed to doing it. Here are just a few baby steps towards making a trip to Mars happen.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post