An Einstein ring can be seen in this image from the COSMOS project. An Einstein ring is a complete circular image of a background galaxy, seen when the background galaxy, a massive foreground galaxy, and the viewer, in this case the Hubble Space Telescope, are all aligned perfectly.
This lens is one of the diverse 67 gravitational lenses found in the COSMOS survey. The lenses were discovered in a recently completed large set of observations as part of a project to survey a single 1.6-square-degree field of sky (nine times the area of the full Moon) with several space-based and Earth-based observatories.
Gravitational lensing is a phenomena where light travelling towards us from a distant galaxy is magnified and distorted as it encounters a massive object on its path. These gravitational lenses often allow astronomers to peer much farther back into the early universe than normally possible.