Grant Erwin writes:

Craigslist is a free online classified ad setup. To find the craigslist nearest
you, visit http://www.craigslist.org/about/cities.html
[...]
In fact, I haven't figured out their
business model yet. No advertising, no fees for buyers or sellers, no nothing at
all.

They do charge for help-wanted ads in SF, NYC, and LA.
Apparently this is lucrative enough to finance the rest of the
operation.

Craig Newmark, the founder, has a guest column in the August
issue of Tecnology Review.