iPhone: SpotWorld is a social travel guide app that lets its users both contribute to a pool of recommended places to visit in a given location and also build itineraries from those locations. If you're planning your next trip and want suggestions from experienced visitors and locals, you can put together a solid itinerary in a few minutes.
To use it, you choose the city you're visiting and take a look at the different sites others have recommended, rated, photographed, and described. If you're just looking for a place last-minute, you can use the location-based "Near Me" feature to find a place to check out that's not to far from you. If you want to create an itinerary or borrow one of the popular ones offered for each city, you'll need to create an account or login with Facebook (which is really the only drawback, if you're adverse to signing up for yet another account). Aside from that, all the other features you can just use and the app won't cost you anything at all. It might come in handy on your next vacation.
SpotWorld (Free) [iTunes App Store]