If you own a HTC HD2 touchscreen smartphone with a 1 GHz Qualcomm Snapdragon processor, a large 4.3 inch 480 x 800 pixel display -- despite having such a high-end hardware under the hood, the HTC HD2 still runs Windows Mobile 6.5. Now, using the RunDroid you can dual-boot Android and Windows Mobile on the HTC HD2.
All you need to do is -- just download the app, use a file manager in Windows Mobile to locate and run it for the first time, and then set it to autorun at boot. And that's it. From that point on, every time you boot your device, you'll be presented with a choice of running either Android or Windows Mobile.
Android needs to be on the SD card for this to work, and Windows Mobile on the phone's main memory. A neat fact is that you can even have multiple versions of Android on the same SD card apparently, and RunDroid will detect all of them and let you choose between them at boot.
To get RunDroid, head over to the XDA-Developers forum.