From OpenStreetMap Wiki
|operator = Stadtmobil|
|Stadtmobil car sharing station (in Germany).|
|Used on these elements|
|Tools for this tag|
|This tag page is a stub. You can help OpenStreetMap by expanding it.
Similar pages for structural reference: Tag:railway=tram, Tag:waterway=dock.
Overview of usage (with taginfo):
So it seems that operator=Stadtmobil is the most widely used operator of car sharing in Germany.