Dopplr was a free social networking service, launched in 2007, that allowed users to create itineraries of their travel plans and spot correlations with their contacts' travel plans in order to arrange meetings at any point on their journey. Additional features included allowing the user to calculate the carbon footprint their journeys have produced. The site was named after Christian Doppler, discoverer of the Doppler effect. The company was based in the "Silicon Roundabout" area of London.