Geoff Keighley's "The Final Hours of Portal 2" app was originally an iOS-only look at the development of Valve's puzzler, but it's moved on to Steam as well. You can pick it up for $1.99 at the Steam store, and it includes the following:

Play with portals in an interactive diagram where you learn how portals work.

360 degree panorama photos of the Valve office and design labs.

Listen to the songs that inspired the Portal 2 development team.

Hear Jonathan Coulton's Portal 2 song in various stages of development.

Puppet Wheatley in an interactive experience.

Interact with fans and voice your opinion via polls and a feedback form.

Check it out over at the Steam store if you're in need of a copious amount of Portal 2 information.