[Xanthe]

You can install third party apps via Google Play. It will list the ones that will work on the Nook HD if you access GP via the device or are logged into a Google account when you access Google Play online, once you've registered your device with Google (I used one of my Gmail accounts).

While most of the offerings seem to work with the Nook HD, there are some that don't. I read somewhere (on MR perhaps?) that apps that need specific GPS access don't work because the Nook only provides location via the WiFi access point.

If you want access to every possible Android app, then you'll either have to root the Nook, or add the Clockwork Mod with the Unidentified Sources mod, or else buy one of the prepared SD cards that already have the Cyanogenmod Android version on it and which allows you boot into Android without rooting your device.

Personally, with the addition of Google Play, I haven't bothered to root my Nook HD because it basically does everything I need.