I used couchsurfing.org while I was in Europe last year. I met a lot of really nice people!
Advice on using the site: have plenty of information on your profile, and when you make contact, say why you'd like to stay at their place (ie. why you want to meet these people - saying "I need a place to sleep" isn't enough). And as the website will advise you, always have a 'Plan B' such as details for a hotel or backpackers, in case you can't find a host. :) Good luck!