The most valuable piece of information, that a tenant possesses, is their move out date. Usually they give it away for free and a lettings agent (leasing agent in the US) would monetise it by charging a landlord fees for re-letting.
But what if tenants held onto that information, and sold it to a 3rd party? The ramifications are mighty interesting.
Renthackr, a website in the US, asks users to give the site advance notice of when they will move out, so it can facilitate the next person coming in. The data is given for free, but it stands to reason that Renthackr would have a whole lot of people banging their door down to pay for this very valuable information.
Their map interface presents 'forecasts' of when units in buildings are likely to become available (currently New York City is where most inventory exists on the site). There's an argument that a lot of these forecasts could be generated with good use of Open Data (more of that in the UK, than the US). But the crowdsourced approach is very empowering. Humans do a lot better than computers at cleaning up data.
Take a look at this Renthackr pitch from New York Tech Meetup (NYTM) earlier this year:
And check out the site itself: http://www.renthackr.com/
With a little bit of funding to spread the message, the crowd really could hack the rental market. One to watch.