All your BASE are belong to CIS
While I wish AaaaaAAaaaAAAaaAAAAaAAAAA!!! every success, I can't let the "there's nothing like it" claim in the feature list go unchallenged. For the last two years Russian outfit Digital Dimension Development have been quietly peddling their own flying squirrel sim. BASE doesn't have surreal skyscrapers, smashable panes or hand signalling, but it does have aerodynamic realism, a scary sense of altitude, and a demo that lets you hurl yourself off the top of Moscow's loftiest landmark.
It's a while since I played the trial, but I do remember getting a lot of mileage out of it. Though there's only the one location, there are eight different challenges on offer ranging from the relatively simple - alight without turning yourself into a fleshy concertina - to the devilishly tricky - land on the roof of a distant tower block. Weather and equipment choices are reassuringly significant. The game even lets you select RPG-style skills and choose jumper weight (...within reason. Skydiving sumos are, sadly, not permitted.)