Dopplr

Dopplr is an online service that lets frequent travelers share future travel plans with friends and colleagues. It also reminds you of friends and colleagues who live in the cities you're planning to visit. The name "Dopplr" comes from Christian Doppler, a 19th Century scientist who came up with what's now known as the Doppler effect. The company claims that Dopplr's users have already shared 110 million kilometers (70 million miles) of trips to over 2000 cities around the world.