AirBnB.com is a bed-and-breakfast booking engine for people's guest bedrooms all around the world. It appeals to people on a budget, and people who prefer to stay with "real people" when they travel (as opposed to hotels). Stephie and I found out about it, listed our guest bedroom for $50 a night, and we are hosting our second guests tonight. Travellers can search by city and price, read reviews, see pictures, request to stay, and pay for their stay at AirBnb. AirBnb then releases the money to the hosts the day after check-in. Ruby on Rails is a great tool for building this site because it's very database and relationship (guests, rooms, bookings, hosts...) driven.