David Pogue in his Nexus One review last week:
The Nexus doesn’t come with any iTunes-style companion software, either. Enterprising techies know about the free DoubleTwist program for Mac or Windows, which simulates iTunes for the purposes of loading up your phone with music, photos and videos.
Google doesn’t supply any equivalent to Apple’s iTunes or the BlackBerry media-syncing software. However, the third-party program doubleTwist, available at doubletwist.com, is designed to function as a sort of iTunes for syncing Android, Palm and BlackBerry devices.