Couch Surfing
I always forget about couchsurfing.com but it's existed since about 1999 and is a place to go if you want a place to stay.
It's particularly useful if you're in or planning a trip to a foreign country and in need of free accommodation. There are 1000s of people around the world signed up on the site and verified members of Couch Surfing, ready to host you for the night. Or two. Or more.
When I lived in a collective in Oslo, the landlady would list any free rooms available on couch surfing and random folk would just turn up and stay. It was actually pretty cool.
Of course, it comes with the obvious risk of meeting a stranger and trusting to stay in their house overnight. I'm sure there are terrible tales of where it's all go wrong, but the Couch Surfing site try to do their due diligence when it comes to verifying the identity of hosts.