Since acquiring an Olympus E-P1 in 2009, I have developed quite an extensive collection of legacy lenses to use with the camera.
One of my favorites, and also one of the more interesting ones, is the SMC Pentax-A Dental Macro 1:4 100mm. An excellent lens by itself, this unit is part of a kit that includes a ring flash, and two matching close up adapters. The lens is K-mount, and appears to be dust sealed; a very high quality lens.
The flash unit can only be used as a manual unit with micro 4/3 cameras, and is rather bulky to use attached to the E-P1, but it works extremely well off camera, in conjunction with a wireless remote.