Cheshire Cat @desc@This group of galaxies has been nicknamed the "Cheshire Cat" because of its resemblance to a smiling feline. Some of the cat-like features are actually distant galaxies whose light has been stretched and bent by the large amounts of mass contained in foreground galaxies.
Abell 370 @desc@A galaxy cluster located about 6 billion light-years away from Earth, in the constellation Cetus. Its core is made up of several hundred galaxies.
Abell 2218 @desc@A cluster of galaxies about 2 billion light-years away, in the constellation Draco. Acting as a powerful lens, it magnifies and distorts all galaxies lying behind the cluster core into long arcs. The lensed galaxies are all stretched along the cluster center, and some of them are multiply imaged.
Abell 2744 @desc@Pandora's cluster. A giant galaxy cluster resulting from the simultaneous pile-up of at least four separate, smaller galaxy clusters that took place over a span of 350 million years. It contains gas so hot that it shines in X-rays. Dark matter makes up to around 75% of the cluster's mass.
MACS J0416.1-2403 @desc@Very massive cluster of galaxies, located roughly 4 billion light-years away. It contains a significant amount of dark matter, which leaves a detectable imprint in visible light by distorting the images of background galaxies. 
RCS2 J032727-132623 @desc@The brightest gravitationally lensed galaxy is seen in the form of bright arcs surrounding the core of this galaxy cluster. The lensing effect increases the apparent brightness of the distant galaxy by a factor of 3.
Bullet Cluster @desc@Two colliding clusters of galaxies at a distance of 3.7 billion light-years. Strictly speaking, the name "Bullet" refers to the smallest subcluster, moving away from the largest one. Gravitational lens studies of this and other cluster collisions are claimed to provide the best evidence to date of the existence of dark matter.