Craigslist: Look here if you want to stay strictly local. Click the 'barter' link in your local 'For Sale' category, and you'll find a wildly diverse range of items and services (legal, of course) that are right in your neighborhood.
Freecycle.org: Though a bare-bones site, Freecycle is efficient, and a perfect way to get rid of unwanted stuff--or to score, for free, something that your neighbor is dying to ditch.
Swap.com: This site has more than 500,000 users and 1.5 million items and services. You create an account by providing details such as your location and what you want to barter. You then upload photos of your treasures and go hunting. You and your bartering partner must work out terms and delivery.
This4That.biz: Here you use "barter bucks," which you earn by making trades with other site members. The company gives you 15 barter bucks when you join, and then 2 more a month.