There's always Craigslist. Or these sites, which involve a membership fee:
Digsville.com Members can rate each other's homes after a swap. $45/year
Intervacus.com Specializes in listings of homes overseas. $95/year
Homeexchange.com Guarantees members will find a swap partner, or they get a year for free. $100/year
Homelink-usa.com Updates its database daily to ensure only current members are listed. $110/year