Swap Your Clothes
Swapping clothes is the easiest and most common swapping. It's easy to host one yourself or find one locally. In NYC, join the Five Boroughs Clothing Swap Meet-up group to stay informed of their regular swap meets, which are generally mostly for adults, but have some kids clothing too.

Below we have info about a children's clothing swap this weekend.

Swap Your Books
There are a bunch of online book swap sites now. The one we've used is PaperbackSwap.com. It's easy to get started. Simply enter the books that you have to get rid of, then make your own wishlist or search for titles you want. Earn credits for giving and use them up for getting. The nice thing about this site is it prints you a mailing label with the exact postage already printed on it so you don't have to bother going to the post office, just wrap the book in paper and drop in a mailbox.

Swap Your Children's Toys
There are two major online toy swapping websites that I'm aware of:

Toyswap.com and ToysToTrade.com both use the credit model to let you trade toys and get "new" ones.

You can also start your own toy co-op with your friends and neighbors. You can either buy toys communally and then have them rotate on a schedule or just agree to pass on your own toys every few months.

Or check out the toy swap happening this weekend in Brooklyn

Swap Your Home
I've written about my experiences house swapping on the site before. You can read the full article here. House swapping is easy for New Yorkers because everyone wants to come here, but you'd be surprised, wherever you live here's someone who wants to go there. Homelink is a great site for if you are looking to go to Europe, an Italian villa, perhaps? For traveling within the US, Caribbean and other destinations, we've had good luck with Home Exchange. Both sites have an annual fee, but when you trade your two bedroom for that apartment in Rome, you'll see it's more than worth it.

Swap Babysitting
Start a babysitting co-op with your friends or neighbors. Start by finding like-minded parents you trust to watch your children, then earn credits for the times  you watch someone else's kids and use them up when you go out yourselves. Babysitter Exchange.com makes it easy to set up and keep track of your credits.

Swap with Your Neighbors
Neighborrow lets you share with your neighbors. You can join an existing group or start your own, at work or in your neighborhood. People list what they've got and they you can request it, pop around the corner and pick it up.

Sites to swap everything
Zwaggle is a giant online swap meet for parents with everything from strollers and gear to diapering and, literally, "everything else." Earn "Zoints" when you give something that you can use for shopping.

At Freecycle meets people bring everything from furniture and household items to clothing shoes and toys and anyone can take whatever they want. You don't have to even bring anything to get something, plus they have a local online list serv where you can request things or see what others are trying to get rid of.