How it works: This “classified-type-site” allows you to list anything you have for sale or want to trade at no charge.
Pros: They have a “Free Stuff” category where you can find everything from moving boxes to used dental equipment (hey, I’m not judging) and a “Barter” category that allows you to post your skills or goods for trade.
Cons: Craiglisters are notorious flakes and are not terribly proficient at returning e-mails. It’s always best to speak to someone on the phone before making the trek to their house.

2. Freecycle
How it works: The name says it all. Everything on this site is, you guessed it, free. Their goal is to keep stuff moving throughout the community and out of landfills. Got a rusty BBQ or half empty can of paint? Chances are good that someone wants it.
Pros: Freecycle has facilitated over a million trades since its inception.
Cons: The site isn’t terribly easy to navigate and you must sign-up to browse their selection of free stuff.

3. Favorpals
How it works: Their motto is “A world without money” and right they are.
Trade your skilled labor for someone else’s.
Pros: It’s one of the only sites that converts your unique skills into a commodity.
Cons: You must join before you’re able to see if there are even people in your area to trade skills with. It’s limited to urban communities.

4. I don’t need it, you can have it
How it works: People donate items they no longer have use for.
Pros: Very parent-friendly. There is a lot of stuff on here that’s new or nearly new and they have an international scope.
Cons: They’re limited to big cities and some categories are pretty sparse.

5. Garden Web
How it works: Green thumbs from around the world keep in touch to trade plants and seeds.
Pros: Easy to use and a great opportunity to find illusive strains of heirloom plants.
Cons: You must pay for shipping.

6. Swaptree
How it works: Using their simple form, just enter the bar code on the books, video games, CDs or DVDs you want to trade.  They then apply an algorithm to your item and list thousands you are able to trade for it.
Pros: Very easy to use, lets you make a wishlist for future “wants” and many of the items are brand new.
Cons: You may have to monitor often as popular items go fast."

http://www.care2.com/greenliving/not-buying-it-how-to-trade-the-stuff-you-already-have.html

You can also start a recycling box.  Basically you take a box, throw in a few things that you don't want, and pass it on to a friend.  They take what they want, add some of their own things that they don't need, and pass it on to another.  It's free, easy, and you don't have to deal with strangers.

http://www.paperbackswap.com/index.php also lets you trade books.  You just pay for shipping.

You can also try the local library.  Everyone knows you can check books out from the library.  But not everyone knows that many libraries take book donations and that they sell books for almost nothing.  The libraries near me have a few shelves of books for sale at all times.  Then a couple of times a year they have a really big sale.  Towards the end you can get a whole bag of books for a couple of dollars.  If you're interested, ask your library about it or get on their emailing list.

Trading means someone gets something they want while someone else gets rid of something without putting it in a landfill.  Everyone wins!