Home Exchange Programmes / Home Exchange sites

AirBNB Stay in (or rent out) a spare room
Couch Surfing  - Crashin' all around the world
HomeAway.com - lots of vacation rentals to choose from
HomeExchange.com - you stay in my house, I stay in yours
HomeForExchange.com - Netherlands-based Home Exchange site
VRBO - Vacation Rentals by Owner