Audi has just launched its "Audi on demand" service as a pilot program in San Francisco, which offers a variety of Audi models to rent on a per-day basis, up to 28 days at a time. Even more interesting, though, is how they're doing it: starting, locking, and unlocking the cars all takes place through a smartphone app, with a digital key card as a backup option (you don't want to be left stranded if your phone dies, for instance). Rather than picking up and dropping off your car at a particular location, concierges can drop off and retrieve cars anywhere of your choosing within San Francisco's city limits.
We're not just talking about A4s, either: you can get everything from an A5 for $200 per day up to an R8 Spyder for $1,285 per day. The fancier models have mileage limits of 200 miles per day with extra charges if you go over. None of this is very cheap — you could get something like a Zipcar or even a Silvercar (which offers Audi A4s exclusively) for less, but then again, you're not renting an RS5 or an A8 L because it's cheap.
Also, considering that this is launching in San Francisco first, $1,285 will probably sound like chump change to many of that city's residents.