My family does it all the time. We use homelink.org, which is a fabulous website. Basically, you create an account and describe your home, put up pictures, list amenities and advantages of your location, etc. and people from all over the world can browse say, homes in San Fransisco, find yours, and send you an email, possibly suggesting some date ranges. Or, you can do the same thing with others. My mom gets several offers a week from all over the world (Sydney, Barcelona, Rome, Istanbul, and Amsterdam come to mind), most of which she pays little attention to.
Last summer, we went to Rome for ten days and stayed in an absolutely beautiful and conveniently located apartment while a woman named Sveva, her husband, and there eleven year old son Giulio stayed in ours in Manhattan. It was great, and of course free! We left them an extensive collection of subway maps, restaurant recommendations, activities etc, as they did for us.
Overall, it is a great system. The organization really protects you and has representatives in every country, so there is very little risk involved. People who have swapped before leave feedback for eachother that you can read, and swappers are generally very respectful of your home. My parents have taken a lot of trips like this, and the one I went on was extremely positive. I would highly recommend it. If you have other questions, feel free to ask.