As loyal readers know, we have given high praise to HomeExchange.com. It's recently gotten better with its Passport/Balloon program. You earn balloons by renewing your membership after a year or by letting another member stay in your house while you're there or while you're not there. You then can redeem a balloon by staying in any member's house (not just a member you hosted), and that member will earn a balloon for letting you stay there. So, in essence, it's like a barter-based Airbnb marketplace. While we believe this explanation is crystal clear, if you want an even clearer one, see the link below.
Link to Explanation of Home Exchange's Balloon Program