http://www.engadget.com/2010/06/04/nexus-one-gets-usb-host-driver-from-a-dude-with-an-oscilloscope

For Sven Killig, running Android 2.2 wasn't enough. No sir, this dude
wanted even more power for his Nexus One, so he went ahead and penned
a few lines of code that have allowed his Googlephone to act as a USB
host. As a result, he can now do cutting edge stuff like plugging in
USB keyboards and webcams, as well as more useful things like watching
movies off USB sticks or connecting to a monitor via a DisplayLink
USB-to-DVI adapter. All these features are demonstrated in the video
after the break, and you'll find the downloadable binaries at the
source -- Sven expects them to work on Android 2.1 as well.