Airbnb.com lets travelers find a spare room for two or three nights by renting spare rooms, cots, couches and airbeds in the apartments of residents in 831 U.S. cities, sometimes for as little as $20 a night.

What it does: Helps budget travelers find an affordable room in and expensive city. It also enables people in heavily touristed cities to make an extra buck from rental of a room in their apartment or home to tourists passing through for a night or two (or three).

What it doesn't do: Rent an entire vacation home; offer to exchange your home or apartment for someone else's or offer free-of-charge hospitality in someone's house or apartment. Nor is it a bed-and-breakfast service whereby people take several guests into their home, and often make a business of doing so.

Plus/minus: Includes traveler reviews. Rooms have not been inspected by the website.

-- ARTHUR FROMMER