We have for the first time exchanged house through homeexchange.com with a Swedish family living in London. They borrow our apartment and we theirs over Easter.
The key lied as promised under a flower pot next to the door, and we were delighted to see the charming town house that we'll stay in the next five nights.
Tip: Dare to change apartment or house! It costs around 10 euros per month to participate in the Home Exchange so join only when you decided to try find an item to swap with. One can try free for 14 days to see which items are in the system, which ones are interested to swap with the particular country you live in and so on. Once you have found a family to swap with - call and talk to them, talk all the contact details through and so on. Everyone we encountered are honest, nice families but it does not hurt to be extra careful.