Two things I would love to see: 1. Kayak.com for ground transportation. Trains, buses, ferries, etc. are a pain to compare prices and book right now. Admittedly there probably isn't as much of a business case for this. 2. A music site where I could collaborate on a playlist with my friends and listen to the music at the same time. Currently there are sites that let you share playlists, but not listen at the same time…
re #2...where do you see the music being sourced from? Allowing listens on shared music via user uploads would be a legal nightmare and the major APIs only provide 30 second clips.
I would go the user-upload route myself though. Just make it non-trivial for users to rip the songs to their hard drives. By the time you get a C&D (if you do), you may have enough traction to start talking to some of the labels about licensing content.