There is a variety of old legacy lenses and modern electronic lenses from other brands that are not designed for Micro Four Thirds (MFT) bodies. Tinkerers just love using these lenses on modern bodies. It's important you identify the other brand mount so that you can hunt up an adapter to fit that lens on your MFT body.
Olympus OM mount (film SLR)
Olympus PEN-F mount (half frame film SLR)
M42 screw thread (used by Pentax, Praktica, Olympus, Seagull SLRs and heaps others)
Minolta MC/MD mount
Canon FD mount
Pentax K mount (used by Pentax, Ricoh, Seagull)
Konica Hexar
Yashica ML / Contax SLR (made by Kyocera)
Nikon all mechanical
Mamiya mount
Fuji mount
Leica LTM
Leica M39
An unusual mount that is quite popular because it is for CCTV lenses is the C mount. These C mount lenses (there are variations) do not create an image circle of coverage for "full frame" bodies but are quite useable on MFT bodies.
There are also medium format legacy lens mounts
Minolta / Sony A mount
Sony E mount
Nikon electronic (based on the previous Nikon mechanical mount)
Canon EF / EFS mount for their EOS cameras
Note that later lenses lost their mechanical f/no aperture ring
When you search the online stores or ebay, use the mount keywords e.g. "OM MFT adapter"
There are two types of adapters
Adapters with no glass inside and are hollow brass pipes.
Adapters with glass inside that typically implement 0.7x focal length reduction and 1 stop brightening of the exposure. These are called Focal Reducers or Lens Turbo
Illustrated guide to legacy film lens mounts http://rick_oleson.tripod.com/index-99.html