1. The Kepler mission detects the first Earth size planets orbiting a sun like star, Keppler-20e and Kepler-20f, December 20, 2011. The two worlds are much too close to their sun to be habitable, but it is still an amazing discovery.
2. The Dawn spacecraft arrives at Vesta to begin a 12 month campaign to study the second largest asteroid in the solar system, July 15, 2011
3. The space shuttle Atlantis last flight July 8, 2011, marking the end of the shuttle program.
4. NASA announces the endof the Sprit MER mission May 25, 2011
5. After 2,767 sols on Mars, the Opportunity MER arrives at Endeavor crater on August 5, 1011
6. NASA announces presence of flowing water on mars August 5, 1011
7. Russia’s Phobos-Grunt mission to the smaller of Mars’ two moons fails. This would have been a terrific mission. One of the main mission objectives was to return a sample of the moon Phobos to earth, which would have been a great acheivement if the mission had not failed.
8. China launches Tiangong-1 space station 29 September 2011
9. NASA announces SpaceX cargo mission to ISS.
10. SpaceX announces plans to partner with Paul Allen and Burt Rutan to launch a rocket from the largest aircraft ever built

Wikipedia has a more complete list of spaceflight activities in 2011