Craigslist is a free, online classifieds service that's localized by geography. Do you need to sell that old refrigerator this weekend? Put it on Craigslist. Users can post items they want to sell and get responses by telephone or e-mail from people who live around them. It cuts out the middleman and often results in quicker sales than you'd get with other services such as eBay or your local newspaper.
In addition to a free-for-all buyers-and-sellers market, there's also a job board that's populated by many employers who agree to pay the $25-$75 per category to list their openings. There are also house and apartment listings, personal ads, and an active user forum where people can talk about whatever they want.