Japan Exposures is offering pre-converted Contax G lenses adapted to Leica M mount.
The Contax G1 and G2 were premium AF viewfinder cameras with excellent Zeiss lenses. Unfortunately, they arrived on the scene just prior to the dawning of the digital age, which subsequently drove the company (Kyocera) from the field. What this has meant is that the fine Zeiss Contax G lenses have been effectively orphaned, and now sell cheaply because of their near-useless lensmount.
A few intrepid individuals have had their G lenses adapted for Leica M mount, and now Japan Exposures is offering pre-converted lenses for direct sale. They remove the optical cell from the G mount housing and add a rangefinder-coupled focusing helical and 39mm screwmount; then a simple LTM-to-M adapter makes the lenses usable on M-mount cameras. There are a few handling oddities, such as the fact that the apertures don't have click-stops.
Why pay a lot of money for this when there are perfectly good alternatives in native M-mount? Don't ask us. Maybe you have a particular fondness for one of the G lenses? Maybe you just want to be different? Idiosyncratic reasons aside, Leica shooters might be particularly interested in the rectilinear Zeiss Hologon, a modern iteration of a rare classic design that was one of the glories of the G system—and that functions as a true 21mm-equivalent on the M8. See the website for pictures, and prices.