Dopplr is a database for professionals to list and broadcast upcoming business trips and manage contacts in those cities. Users keep a calendar listing the places they will be on specific dates, and Dopplr connects these members with trusted friends from their contact list that will be in the same location at the same time.
Users can search the schedules of their friends, or search a specific city for contacts or tips about that location. Dopplr can import contact information from a variety of sites (Gmail, facebook), and also has a facebook application. Dopplr works on mobile phones as well.
Dopplr.com In Their Own Words
“Dopplr is an online service for frequent travellers. It lets you share your future travel plans with a group of trusted fellow travellers that 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.”
Why Dopplr.com It Might Be A Killer
Dopplr just gets it right. It is a simple, practical idea, is executed right, and is continually thinking about what else can be done to improve the service. Dopplr has also received some pretty major endorsements- Jimmy Wales, of Wikipedia fame, calls Dopplr his favorite non-Wiki website.
Some Questions About Dopplr.com
How long before LinkedIn adds a similar application? 
