I just removed Windows 7 Home Premium and installed Windows 7 Ultimate instead. However, the feature that I had used from the beginning was the ability to actually stick a window to the side of the screen, so that it literally divided the screen.
What I mean is that, when I e.g. put my IM client to the right side of the screen and then open Notepad and maximize it, it will only fill up the space my IM client didn't reserve.
I am 99% sure I did not have any additional software installed for this feature. However, I can't find a way to do it now.