Three of the black cabs roaming Central London this week are offering free rides as a reward for supporting Sound Off For Justice - the Law Society's campaign against the government's proposed cuts to legal aid. The cabs are equipped with iPads and iPhones so passengers can easily sign the petition, join their voicemail protest, watch campaign videos or even follow the campaign's social profiles. They're also being tracked live through Glympse and location updates are posted to the Twitter profile several times a day - offering nearby people who spot the updates the chance to tweet back and 'hail one digitally'. More info here.