Using a simple algorithm, Belokurov et. al discovered this
almost perfect Einstein-ring around a luminous red galaxy in the
SDSS database: They called it the Cosmic Horseshoe. The ring has
a diameter of 10 arcseconds, which counts as large. The lensing
galaxy has a mass of about 5 x 1012 solar mass - about ten times
the mass of the Milky Way! For comparison, here's an other lens,
also from the SDSS, discovered serendipitously, the "8 O'clock
Arc": This is less ring-shaped, it has 3 images of the same
background galaxy on the top and a fourth on the bottom. It
shows a similar size, but the lensing galaxy is estimated to
have a mass of about 1/5 of that in the center of the Cosmic
Horseshoe. A news article. The scientific article.