As I’ve starting travelling a lot recently, I’ve been making reasonably heavy use of both Dopplr and the less well-known TripIt. The idea behind Dopplr is simple: tell it which cities you’re travelling to and it will share that information with your other Dopplr contacts, notifying you when you’re in the same place. You can also syndicate your travel plans - I have mine published on Facebook and available as a feed via Google Calendar.
TripIt, although it has similar facilities for maintaining a list of travelling contacts, originates from a slightly different and more ambitious idea. Essentially, you email TripIt confirmation emails for hotels, airlines, car rental, etc. - and it parses them and automatically organises them into trips with information-rich itineraries, including weather, maps, city guides, etc. You can print those out, but (more usefully) you can again syndicate them into tools such as Google Calendar via iCal. This way, I end up with details about all my flights, hotels, and so on in my calendar automatically.
TripIt’s not perfect - I’ve found a few bugs - and TripIt doesn’t support every single travel agent (for example, it doesn’t support the one we use within IBM - at least not directly). But the TripIt team are very responsive to feedback - I notified them about a Hertz reservation email this afternoon that wasn’t recognised. They’ve already fixed the bug and the information has appeared in my TripIt account. I’m not sure how they plan to monetise their service (although that’s still not clear with Dopplr either), but I am likely to stay a regular user for the foreseeable future.
(Honourable mention for another useful travel website goes to Kayak, which has the most flexible and useful flight search interface I’ve found).