Craigslist is an online host for classified ads that can be found at http://www.craigslist.org.
Craiglist works on a local level, with subdomains ranging from San Francisco and New York, from Austin to Boston, and of course, my hometown, Portland.
Classified ads can be placed for free, sorted by category, for everything from housing to jobs, furniture to sex. Subcategories help you find specifics on what you're looking for. The website also maintains a list of community events (such as The Pushovers performances and classes and so forth) and active discussion forums.
Yet the (arguably) most famous part of Craigslist is the Rants 'n' Raves (save for possibly the Casual Encounters section), a list of posting in which people can flip out and complain about whatever they want. While this may sound like a recipe for disaster, the Rants 'n' Raves section of Craigslist (along with the rest of the site) is policed by the community. Spam is quickly flagged and brought down, while funny posts are voted up and kept in the Best-Of-Craigslist archive.
The website was started by Craig Newmark in 1995, and was originally intended to be named sf-events. The website has grown over the years, and is now an international phenomenon, getting written up in dozens of magazines and newspapers.