I've used Couchsurfing extensively here in the states and in Europe. I have nothing but great things to say about it, I think it's a fantastic service. The reference feature usually tells you almost everything you need to know about someone, as you can even look up the people that left the references. It's really nice to be able to have a local connection almost anywhere you're visiting in the country. I think it's making the world a smaller place(in a good way).

I haven't used warmshowers, but I'm very supportive of well-thought-out social networking sites like this. I've met several people I still keep in touch with on Couchsurfing.

My 2 cents.