The short registration distance of Micro Four Thirds (20mm from mount to sensor) makes it possible to adapt a wide variety of SLR and rangefinder lenses for use with this format without a need for interposed optics.

Vivek Iyer has been experimenting with a homemade Leica M to M43 adapter and producing excellent results with a Canon 50mm f/0.95 rangefinder lens on his Panasonic G1 (link).

A recent issue of Nippon Camera contained the following photos of various lenses adapted to the G1:

Source: Photofan.jp forums

The G1 provides a magnified EVF view of the central portion of the image, which allows for precise manual focusing. I'd like to see Panasonic drop the vestigial rangefinder-like dependence on center focusing and allow the user to select any of the 23 AF points for magnification when focusing manually.