Scientists at the European Southern Observatory (ESO), using the highly successful HARPS (High Accuracy Radial Velocity for Planetary Searcher) spectrograph, have found a planet orbiting the red dwarf Gliese 581. Extrasolar planets (ie. those orbiting stars other than the sun) are discovered with great frequency these days, but this one, called Gliese 581 c, is special.
The most reliable method for finding extrasolar planets relies on trying to spot a star 'wobbling' as an orbiting planet exerts a gravitational pull on it. This method is heavily biased towards finding planets that are both extremely large and extremely close to their parent star, and a good chunk of those extrasolar planets we've found so far have fallen into this category. Gliese 581 c, on the other hand, is a mere five times the Earth's mass, and 1.5 times the Earth's diameter - the smallest extrasolar planet we've yet been able to detect. Even more striking, it is the right distance from its parent star for liquid water to exist on its surface.
Although there is a lot about Gliese 581 c that we don't yet know, this is the first planet we have ever discovered that is 'like' Earth. Just how much like Earth, I imagine will be the focus of much research over the coming years.
Astronomers Find First Earth-like Planet in Habitable Zone - Read the press release on the ESO homepage.
Most Earth-Like Planet Discovered Orbiting Nearby Star - an excellent article on the discovery at the Planetary Society, including a discussion of how astronomers were able to detect this planet.
HUGE NEWS: first possibly Earthlike extrasolar planet found! - Phil Plait has a nice discussion of this discovery, and the possibilities it embodies, at Bad Astronomy.