Freecycle and craigslist are two great resources for passing along free (or inexpensive) items. And I'm still experimenting with Zwaggle, which allows you to trade children's items, with mixed results.
But what do you do with those piles of dust-collecting books or abandoned CDs?
A few Web sites exist that key into your hobbies and allow for free to low-cost swapping.
By sharing your items with other interested readers or listeners, you earn credits that you can redeem for swapped items of your own. While you're responsible for the shipping costs, you also don't have to purchase these items new.