Is there a list of free accommodation resources?

Couchsurfing and Hospitality Club are the most common, for sure. However you can also consider bar work - in some countries (eg England) it often includes accommodation. Similarly, working in a hostel often scores you a free bed as well. And sometimes just chatting to friendly locals may work in your favour.

CouchSurfing is the best choice. Also there is http://hospitalityclub.org, but it is going to stagnation from year to year, and http://bewelcome.org. Try it all, and you'll get the variant to stay.

You can try http://www.airbnb.com/. I have used the site to figure out accommodation for some of my travels and have found it pretty useful.

The downside, you cannot book an accommodation right away. You need to communicate with the owners and see what works for you. This would mean you start your search pretty early.

Sometimes you could be interested in sleep in your arrival airport so you can save the money for the first night in a new place. There is a page that will help you to decide if one or other airport is more convenient to sleep there: Sleep in Airports.

NOTE: It could sound crazy, but I've sleept a lot of times in different airports and if you have no problems to sleep with some noise, that's a good option if your flight departs too early or you arrive too late to your destination.

You can try the subreddits of individual cities, and ask to crash over. I've had far better experiences off of Reddit than off of Couchsurfing, perhaps because people often "stalk" my Reddit posts before offering me their couch, which means that they're often more interested in me.

Couch Surfing sounds good, but if you like luxury, there is a luxury version of couch surfing: house sitting. I know two websites that offer house sitting: Houses it world and Care Taker. The idea is the owners offer free accommodations in luxury or holiday houses and you clean and guard their homes.

The only problem is that for subscribe to these sites you have to pay a small fee.

Also you can exchange your home, like Cameron Diaz and Kate Winslet in the film "The holidays". That's an option ideal for families due you will save a lot of money and you have not to work. The problem is you have to be very confident with your guest. Use ExchangeMouse to find homes to exchange among 60+ home exchange clubs. Results vary heavily depending of your location and destination as interest needs to be mutual, tough there are sites offering points based exchanges to get beyond this requirement too.

As a host for both Helpx and WWOOF, I ask that anyone using those services do not treat them as free. It can cost dearly to host someone, especially in a remote area where they have to be fetched and then returned to some mode of transportation.

There are lots of opportunity for fantastic experiences, for both the host and the volunteer. There is nothing worse than having someone waste my time and energy by taking advantage of the situation, and behaving as though it is a vacation. Only use those services if you have the intention and integrity to be productive and self responsible. It is NOT a babysitting service...it is a job and has basic commitment levels to fulfill.

House-sitting is an option for longer-term stays, and a really good way to experience a place more as a “local” than a tourist. I found MindMyHouse a good place to start to find opportunities.