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 Err… WinMo 6.5 is a little bit outdated.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.
HyperDroidGBX (Gingerbread Xtreme Edition) is the first open-source custom ROM designed and optimized just for HTC HD2. It is cooked by XDA Developer, pongster. I was staying away from his builds all the while because of LCD Density at 167 by default. For me, all the stuffs are way too small on this density. So I changed it back to 240. By the way, this ROM is based on Android 2.3.3 Gingerbread and using Tytung r8.3 Kernel.