For those who need examples of P2P-based business models,  marketing examples, and civic examples, here is a list that I will continuously update:

Housing

• Roomates.com - A roomate finder and roomates search service which covers thousands of cities nationwide.
• Screening for Gold: How to Find and Keep Your Good Housemate
• How to Start a Housing Co-op - one of the best affordable housing options around, and shared food expenses and cooking can increase your savings.
• Guide to Sharing a House - buying a home by yourself may be out of reach in high cost areas, but shared ownership might be the ticket.
• Cohousing Directory  - Cohousing is homeownership in a neighborhood that shares.
• Craigslist - find almost anything including a house or housemate on Craigslist.

Food

• Gobble, Grubwithus, Feastly, and Eat With Me (Australia) are Airbnbs for meals. Use them to find or host a meal in your neighborhood. Never eat alone!
• MamaBake - Large batch group cooking saves time and money, not to mention it’s fun!
• Local Harvest - A massive directory that helps you find farmers’ markets, CSA’s, and other sources of sustainably grown food in your area.
• Neighborhood Fruit - find and offer free fruit to your neighbors with this site and iPhone app.
• How to Share a Vegetable Garden
• How to Start A Farmers’ Market
• Host a Baby Food Swap

Personal Finance

• Lending Club - An online financial community that brings together creditworthy borrowers and savvy investors so that both can benefit financially.
• Zopa (UK) - Where people get together to lend and borrow money directly with each other, sidestepping the banks for a better deal.
• Prosper - A peer-to-peer lending site that allows people to invest in each other in a way that is financially and socially rewarding.
• SmartyPig - social savings bank that enables you to save for specific goals and engage friends and family to contribute.
• How to Save Money by Sharing

Entrepreneurship / Work

• Kickstarter - A crowd-funding site powered by a unique all-or-nothing funding method where projects must be fully-funded or no money changes hands.
• BetterMeans - Use open-source decision-making rules, and self-organizing principles to run your real-world projects.
• TaskRabbit - make extra money doing odd jobs or outsource small tasks and deliveries  with this errand and delivery marketplace.
• Skillshare - earn some extra cash teaching a class or learn a new skill to help you get or create that dream job.
• Use the coworking wiki, Loosecubes or Liquidspace to find a friendly place to cowork. Coworking is a flexible and community-oriented workspace option for business travelers, independent workers, and entrepreneurs. 
• How to Find a Job Using Social Media
• The Shareable Job Search Search
• How to Start a Coworking Space
• A Guide to Casual Coworking - Why not cowork anywhere? Here’s the definitive guide.
• How to Create Your Own Green Job
• How to Make A Franchise Shareable

Travel

• CouchSurfing - An international network that connects travelers with free accommodations offered by locals in over 230 countries. There’s no better way to immerse yourself in the local culture than to stay with an friendly local.
• Airbnb - the leading a fee-based service that connects people who have space to share with travelers looking for lodging, all over the world. Also check out similar services iStopOver, Roomorama, and Tripping. Save a ton of money and connect to the local scene through these peer to peer lodging sites.
• How To Swap Cities - a guide on how to swap offices with someone from another city inspired by SwapYourShop.
• Try out Vayable or Guidehop for tours and experiences created by independent locals for those seeking authentic experiences.

Land / Gardening

• HyperLocavore - Share gardens, seeds, tools and good times growing food!  See similar services Shared Earth and UK-based Landshare.
• Tool libraries - check out this handy directory of tool libraries.
• How to Create Your Own Seed-Lending Library
• How to start a Crop Mob - Crop mobs allow you to get and give gardening help.
• How to Share a Vegetable Garden

Transportation

• Check out these top transit apps for car free living.
• Carsharing directory - find carsharing service providers in your area with this international list.
• Zimride, eRideShare & Carpooling.com (Europe) - Find a ride or offer a ride on these top ridesharing platforms. And check out Avego for real time ridesharing through a mobile app.
• ZipCar - the largest fleet-based carsharing service in the world.
• Spinlister - Rent bikes from neighbors
• RelayRides, Getaround, Just Share It, and Whipcar (UK) - Rent cars to or from neighbors using the leaders of the peer to peer carsharing movement.
• Weeels - order cabs and share rides with this smartphone app.
• Park at my House - Provides affordable and fine-free parking by enabling property-owners to rent out their empty driveways, garages, and car parks to drivers needing somewhere to park.
• How To Share a Car With A Stranger
• How To Be a Carfree Family

Media (Books, Movies, Games, Music)

• Swap.com - The leading online swap marketplace for books, movies, music and games.  Amazing selection. Update: now swapping everything.
• BookMooch - Lets you swap books you no longer need in exchange for books you really want.
• Goozex - A trading platform for video games and movies.
• Paperback Swap - Trade paperback books for free. Also DVDs and  CDs.

Clothing

• Check The S.W.A.P. Team, ClothingSwap.com, Swap for Good, and The Swapaholics for clothing swaps near you.
• Or host your own swap using this guide, How to Throw a Community Swap Meet.
• Use Renttherunway to rent authentic designer clothing for up to 90 percent off retail prices.
• Swapstyle - An fashion website where members can swap - rather than buy - unlimited designer clothes.
• Try Bag Borrow & Steal and Fashionhire - to rent designer handbags and accessories at affordable prices.
• Check out 99 Dresses where you get access to free fashion in “the infinite closet” by sharing your unused clothes.
• And when the time comes to start a family, use ThredUp to swap kid’s clothes, books, and toys with other parents.

Redistribution Sites (where unneeded stuff finds a loving home)

• Freecycle - The original grassroots organization for giving and getting free stuff in your town.
• Craigslist - This is the ultimate free classified site with categories for free stuff, shares, barters, sublets, garage sales, house swaps, tons of used stuff for sale, and more. New in town? You can set yourself up with a job, an apartment, furniture, and a date all from this site.
• eBay - International online auction that allows you to buy from and sell to other individuals.
• Try out Zaarly, a classified service optimized for smartphones.

Renting and sharing of general goods where you live

• Rentalic, Neighborgoods, Keepio, SnapGoods and Zilok (US & Europe) are leading peer to peer rental and sharing marketplaces.
• Do you want to co-own something with friends or family? Jointli and Sharezen are the perfect tools to buy, use, and manage a shared asset like cars, boats, planes, tools, real estate, and more.

Campus

• Chegg - Rent expensive textbooks on the cheap.
• Better World Books - Save big on used textbooks.
• CafeScribe - A new service that lets you download electronic copies of your textbook, add friends, and share your notes.
• GradeGuru - A leading student notesharing and social network.
• Free Technology Academy - free college classes on open source technology and standards.
• Open Courseware - free college course materials offered by scores of top universities from around the world. Also check out MIT’s free classes.

Production, Manufacture

• The Italian Arduino project (electric engineering)
• Ronen Kadushin (furniture)
• SketchChair
• FurnLab
• CubeSpawn

Medical

• Open Prosthetics Project It was started by a former soldier who had lost a hand in war and was unable to find a commercially available prosthesis suiting his needs. A special goal of the project is to improve the medical treatment of people who cannot afford to pay a lot, e.g., in the Global South.

Clothing

• Openwear is a collaborative clothing platform that supports people in becoming producers and finding collaborators.

Other Guides:

• Check out the Mesh directory by The Mesh author Lisa Gansky and the folks at Mesh Labs. Features thousands of searchable entries organized by category.
• And here’s author of What’s Mine is Yours: The Rise of Collaborative Consumption Rachel Botsman’s concise list of leading services.