Simple and brilliant - a really clever advertising campaign on the tube and a very effective way of measuring ROI. The company in question created a clear message and a unique url, allowing them to precisely measure traffic generated by the campaign - brilliant!

As someone that used to flog the occasional bit of advertising space (for about 10 years) I witnessed the gradual shift from print to digital media. From a simple - "call this number to buy" on a printed page - to a whole lot of head scratching about assorted web campaigns, really great to see such a simple and elegant approach!