Have you seen the movie, "The Holiday"?  It's about 2 women who go on vacation and swap houses with each other.  There really is a website that helps people arrange that! 

 

   You pay a flat membership fee for the year and you can swap as many times as you like.  If you don't find a swap in the first year, your 2nd year is Free.  Since lodging can be one of the most expenisve parts of taking a vacation, this could be a real money saver!