Yahoo! Travel has released a nice little planning helper called Trip Planner. It’s somehow a mashup of their own services (Yahoo! Travel, Yahoo! Maps, also leveraging Flickr somehow) with a shot of 37signals minimalism.
I don’t want to duplicate the infos given in Yahoo’s tour here (which tells all), but you basically can create trips, add items (Yahoo helps you starting out by offering suggestions from its Travel Guide), and organize your items (by category, you can schedule them, tag them, and annotate them), and share your trip with the world or a list of friends.
I just planned my next trip (to Vienna) and it really was fun to use.