In arguably one of the best new ideas to come up out of the web 2.0 wave, Swivel lets you view and compare data - any data that is comparable. Of course, users will probably upload huge amounts of useless data, but there are already lots of datasets online that show that this will be a tremendously useful service.
The data you upload is public and available to everyone - unless you pay a fee; then you can keep it private and compare it against public data.
While I don't expect enormous popularity of Swivel (for example, of YouTube magnitude), this service will be invaluable to professionals. I, for one, can already get some very valuable info here, for example, the number of subscribers for different MMORPG games.
It will be interesting to see how big analyst companies, like IDC or Gartner will react if someone publishes some of their data; although I don't expect to see a huge wave of lawsuits here, I'm sure some of these companies will be less than pleased.