HTC HD2 has got lots of unofficial development. Born with Windows Mobile 6.5, developers got it running Android (up to KitKat!), Windows Phone 7 and some Linux distributions.
Most of the credit goes to the Dark Forces Team, some great developers that made the HD2 run also Windows Phone 8 and Windows 8 RT; even if they didn’t release anything.
Following one of the members on Twitter, however, I found that they were also working on another great operating system: iOS 4!
It seems that they never got it fully working and, now that they moved to HTC One, iOS is probably abandoned. However, there are two photos of an HTC HD2 installing (trying to install?) iOS.