Cornell Electron Storage Ring, a 10G eV electron-positron colliding synchrotron located on the southwest portion of the Cornell campus in Ithaca, NY.
To give an idea of the scale of the thing, the synchrotron runs basically the length of a football field. I know this, because it is located directly under one. The ring is 768 meters in circumference, and is home to bunches of electrons and positrons completing some 390,000 revolutions every second (that puts them at 99.9999995% of the speed of light).
The synchrotron is being used for a variety of high energy and particle physics experiments, most of which are studying rare hadronic decays, where heavy particles not usually seen in nature are created in the wake of electron-positron collisions. These events are studied by tracking the resulting particles that come out of the decays through a number of layers of sensory systems collectively known as the CLEO detector. CESR also participates in the CHESS program, which conducts experiments using the X-rays created as a bi-product of the process of accelerating electrons and positrons through the rings of the synchrotron.