via Livescience, a good article on exoplanets, and what we've discovered so far about them. Most notable is the smallest yet found, Gliese 581 C, which is about only 5 earth masses and could hold liquid water. In total, over 200 planets have been discovered orbiting other stars so far, with more being observed almost every week or so it seems. Around 25 systems have been found that hold multiple planets like our own solar system. The goal of course, is to find another planet like our own.
"The ultimate goal, say many planet hunters, is to find Earth-like planets, or those with similar masses, orbits and rocky compositions to Earth. And beyond finding the physical Earth-like attributes would be to find life. So far no "Earths" have been identified, though observatories are coming online with the sensitivity to detect small objects that orbit far from their host stars, as our planet does."
There are two principal methods of discovery, the "wobble" method, where the gravitational influence of a planet is observed in a star's light reaching Earth, and the "transit" method, where a planet crossing it's star's light reaching us here on Earth causes an observable (to a computer, anyway) decrease in the light reaching us. The wobble method has been the most successful in finding planets as the transit method depends greatly on all the astronomical bodies lining up just right. Even so, around 20 planets have been discovered using the method.
I just know the Vulcans and Klingons are out there waiting for us.