Been playing with Dopplr.com since crucially sent me an invite a few weeks ago. Really like the concept, they do one simple useful thing which is showing if you have overlapping trips with your friends. anildash has usability issues with it, though I'm hoping it will improve since it is just alpha. Does illustrate how powerful it is to have open source software since if Dopplr were, then all the geeks which are using it could scratch their own itches. For now I just leave you with a screenshot.