Dopplr is a site that keeps track of people's travel plans. Here at Global Web Management in ABB we use it to keep track of who is where, as most of us are nomads by profession or inclination. Enter your destination and the relevant dates. You decide who should be able to see your information, and they may or may not let you share their.
The application alerts you to coincidences, so if someone you know happens to be in the same place you have a chance to get together for a beer in Berlin or a cuppa' in London. I personally prefer to paste their RSS feed into my Netvibes page, but you can also subscribe to e-mail alerts if you want.
Until yesterday the site was available by invitation only, but they have now opened up to everyone who wants to join. Recommended!
UPDATE: I forgot to mention that Dopplr also comes with a Facebook widget, so your whereabouts can be posted to your profile for your friends to see. Through the widget you can easily add trips without having to make the detour to the Dopplr site itself.