The other day I stumbled upon the "Conference Bike" (aka CoBi), a strange human-powered vehicle created by Eric Staller.
It's not a bike at all but it is pedal-powered, with 7 seats.
It's fun to ride but very heavy (made of steel - please make the next one in aluminium!). So not really ideal in San Francisco (virtually impossible to go up-hill). But it's a very social experience to ride it. It's like a self-propelled party on wheels.
Apparently the vehicle is street-legal (probably falls in the "cycle" category for Traffic Laws?).
The Conference Bike is commercially available, so check out the website for more info, photos and videos: www.conferencebike.com/