HTC HD2 is a great phone. You can run stock Windows Mobile 6.5 on it. Err… WinMo 6.5 is a little bit outdated. No worry, you can install custom ROMs like Android and Windows Phone 7. Personally, I don’t recommend WP7 unless you need to use a killer app only build for it (but I doubt that). So Android OS is the one that you should get. And here are the latest and greatest Android ROM based on Android 2.3.3 Gingerbread that you should try.