Saul Perlmutter, Brian Schmidt, and Adam Riess have shared the 2011 Nobel Prize in physics for the 1998 discovery that the expansion of our Universe is accelerating. This acceleration is what led us to conclude that most of the energy density of the Universe is made up of dark energy, although in the time since then there have been independent observations that point to dark energy.
My favorite comment was the one made by Martin Rees, who wishes that the Nobel Prize could be shared by more than three people, as the two groups deserved recognition in addition to the leaders of the two groups. Indeed, in 2007, the Gruber Prize in Cosmology was divided four ways: Saul, Brian, Saul's Group, and Brian's Group. (I thus shared in the "Saul's Group" part of the prize.)
Here is a link to a talk that I gave in Second Life a couple of years ago about the discovery of the accelerating universe. An audio recording of the talk is online, in addition to slides in PDF format.