A global housing listings website that's integrated with Google Maps. It lets you post/find for FREE any kind of listing (apartments/houses for rent and sale, flatshares, housing swaps, vacation rentals, office space...).
You can search by the usual criteria (rooms, price...), but also graphically by area.
It's also integrated with RapLeaf, a portable reputation service, to help you spread your reputation as a flatmate, swapper, tenant or landlord, so that you can rent/buy/sell/swap your housing (or whatever) more easily in the future.