1. Craigslist.com. If you list on only one site, choose Craigslist. This site is free in nearly every city and has more traffic than any other classified listing site. Myself and my clients have had tremendous success with Craigslist, even though we do have to put up with a substantial amount of spam.
2. Postlets.com. As far as I know this is the top real estate posting template site for Craigslist ads, and it also syndicates your listings to a fairly large number of other classified and real estate websites.
3. Backpage.com. While this site does not get as much traffic as Craigslist, it is growing quickly. It is similar to Craigslist, and quite easy to use. It also has paid options to make your ad more visible to potential clients.
4. Hoobly.com. I have noticed that many of my classified ads on this site rank well in Google searches, and it gets a substantial amount of traffic.
5. Kijiji.com. I use this for the same reasons as Hoobly, mentioned in #4.
6. Zillow.com. This is becoming one of the most trafficked realty websites, and has the option of putting your listings on the site for free. You can also have an agent profile. Again, my real estate ads often rank well in Google searches when placed on this site.
7. Oodle.com. This site is well designed and easy to use. It's very easy to add your listings to it.