Are there any window managers for Android? I once saw a YouTube video where someone had hooked up a bluetooth mouse and keyboard to a Galaxy Note and was manipulating apps like on a desktop computer, where each app had window decorations and multiple apps could be visible at once.

I would like to have such a window manager (preferably not requiring any external devices) for my Galaxy Nexus. There are occasionaly situations where it'd be helpful to be able to see two apps at once.

For example, I might receive complex data in an email that I want to input into another app. However, Gmail doesn't allow you to copy text from an email (which is just rediculous). So, I have to remember a little bit, switch apps, type it in, switch apps again, etc. It would be much better to be able to put my phone in landscape mode, fire up the window decorations, and tile the two apps side by side so I can see the email address at the same time I'm entering data in the other app.

I've searched for a window manager for Android, but I can't find anything, despite the video I saw. Does anyone know of one?
The window manager you saw is part of Samsung Android 4.x O.S. and only works on Samsung Tablets.

There are lots and lots of differences between Samsung Android and Google Android.

I did a quick comparison for the Galaxy S3 smartphone:
here