At launch the site featured dozens of datasets from around 20 Federal agencies ready to be used by the public, commercial and NGO sectors in mash-up applications and services.
The public is also able to suggest additional information to be made available through the site.
I am an extremely big fan of making public sector data available online (where there are no security issues), particularly when the data is readily available for online reuse through APIs, XML, RSS, KML/KMZ and similar machine-readable formats.
A speech by the US Federal CIO, Vivek Kundra launched the site and made the purpose extremely clear in the video below. Kundra expects more than 240,000 datasets to eventually be available online, per the post at Governing People by George Fahey, Data.gov opens.
The post also comments that,
The first applications built on this data has already arrived (see FBI Fugitive Concentration).
This demonstrates how quickly the public can make good use of public information when it is made public.