As some 180,000 people – most of them from outside of the city limits – descend on Golden Gate Park this weekend for the annual Outside Lands Festival, the big question is: Where are they going to park?

After all, the parking passes offered by Outside Lands (at $150 a pop) sold out almost as soon as they became available.

For everyone who didn’t grab one of those, a scrappy little startup called ParkPlease says it has come up an alternative.

Started by S.F. State alum Charlie Ansanelli, and four others, ParkPlease matches those seeking parking places with residents, businesses, schools and churches in the neighborhoods surrounding the park willing to rent out their driveways, garages, or small lots by the day.

“I started ParkPlease because when I used to have a car, parking in the city was such a nightmare,” says Ansanelli. “It was this terrible nuisance that ruined whatever activity I was trying to enjoy.”

After a few months of furious effort in their "living rooms and coffee shops," Ansanelli and his co-founders launched ParkPlease two weeks ago, focused squarely on Outside Lands as its big test case.

“We decided to open it up and see if it works. We want to get a lot of user feedback about what works and what doesn’t work so well,” he explains.

Like most p2p services, ParkPlease requires users to sign in with their social graph (in this case, Facebook), as part of the effort to establish a secure, reputation-based marketplace.

Both those renting out their parking spaces and those paying to leave their cars there will rate each other afterwards, and “our reputation tools will reward good behavior in the future,” Ansanelli says.

He also says that if you should you somehow arrive at a destination you rented only to find it occupied by a different car, “our customer support will find you another spot right away at no cost."

As word of the new service has gotten out in the western part of the city, over 120 parking spots had been listed on the ParkPlease website by mid-day Wednesday.

The company also have partnered with a local elementary school to create a “pop-up parking lot” with around 100 more spaces.

Those renting out the spaces are setting the rates (which appear to range from the $20s to the $90s per day), but Ansanelli says that his team is starting to offer “price coaching” in order to help them identify the most reasonable rates that match the expectations and willingness to pay of those seeking parking spots.

This is part of the p2p market evolution that is fascinating to watch, as supply and demand inexorably find their way into balance.

Once the ParkPlease team has absorbed the lessons of managing concert parking this weekend, they intend to open the service up all over the city, for events like Giants games, or for commuters paying high daily fees at present, or “just when you want to be able to park near a restaurant for dinner.”

“Our mission is to make parking safer and easier,” says Ansanelli. “We want to make it stress-free.”

The company's initial business model rests on charging a small convenience fee to those renting out their parking spaces.

Like other emerging p2p marketplaces we’ve profiled: Getaround, Zaarly, SideCar, Airbnb, LiquidSpace, and Blockboard (which has since been acquired by Klout), ParkPlease is enabling people to share resources and information with each other in new ways – “collaborative consumption,” if you will, or what is called the “sharing economy.”

You can read more about these kinds of startups – and many others – in our new ebook, 30 Startups to Know Now.