Couchsurfing is a great site - I have used it a few times now and have not had a bad expreience. A few sites have started up now which allow you to charge for your room if you are hosting which I think is a bit more secure.
If you use www.crashpadder.com, for example, both parties have to verify their identity and address before making a booking so it's less likely to be a magnet for weirdos!