Android running on an iPhone. A member of iPhone development team called Planetbeing have ported Android to run on iPhone. iPhone can be configured to dual boot iPhone OS and Android phone. I even had a glimpse of Android running on iPhone in one of my friend's mobile. I would rate the experience is quiet better than the Samsung I5700 galaxy spica The graphics transition is quiet faster.
The port is currently available only in the first generation iPhones. Hope it would soon be available in iPhone 3G and 3GS models. The source code is available online here
Have a glimpse on this video.