If I were a frequent traveller I might be interested in this.
Dopplr is an online service for frequent travellers. It was created by an international team of world travellers as a tool for our own use. We liked it so much that we decided to open it up to our global friends.
If you travel more than five times a year and have friends who do as well, then Dopplr is for you.
How does Dopplr work? It lets you share your future travel plans with a group of trusted fellow travellers whom you have chosen. It also reminds you of friends and colleagues who live in the cities you’re planning to visit. You can use the service with your personal computer and mobile phone.
Membership by invitation only. I heard about it via Tim O’Reilly, who says it was the nicest thing he saw at Etech.