I just started experimenting with dopplr after receiving an invite today! It is a web 2.0 application that lets you post details about your future travel plans and is mostly targeted at frequent travelers who want to synchronize their schedules with friends.
I think it can be a really useful app, even though it only gets interesting if enough of your friends (a) travel a lot and (b) join the community. So, anyone who likes an invite please let me know, (the service is invitation only for now).

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