Where is your Tube train right now?
Earlier this month, Transport for London removed restrictions on their travel data.
I know that doesn't sound like an earth-shattering revelation, but it means that anyone can go to the Greater London Authority's Datastore site and make applications such as a live map showing where each Tube train is right now (developed by Matthew Somerville).
Looks like we'll see more of these on our PCs and mobile phones. Are they useful? You'll be the judge.