This is the Olympus MMF-1 4/3 to Micro 4/3 adapter for use with Micro 4/3 cameras such as the Olympus E-P3. It allows standard Four Thirds lenses to be mounted and used on Micro 4/3rds cameras.
Note: While this adapter does allow autofocus with compatible AF Four Thirds lenses, the autofocus is much slower than it would be if the lens is used on a Four Thirds camera.