Ceres is the largest asteroid, almost 1000 km in diameter. From Earth, it has an apparent magnitude of 6.7 at its brightest...a bit too dim to be seen with the naked eye, although even modest binoculars will show it. The Dawn spacecraft will be going into orbit around Ceres in early 2015, giving us rather better pictures than this, the best picture taken to date using the Hubble Space Telescope:
Interestingly, Ceres may have a subsurface ocean of liquid water, leading to the possibility of life there.