* virtual desktops (if you have too many open documents, you deserve a fresh new desktop)
* minimize any window to system tray
* minimize a window to a floating icon on your desktop (not to the taskbar or systray)
* make any window "always on top"
* hide a window completely
* change the opacity of a window
* change the priority of the process that created the window
* add favorite locations to open / save dialogs
For example, you can right-click on the maximize button in the right-top corner of the window to transform a window into a floating icon. The icon occupies little space and it's always available. If you right-click on the floating icon, you can preview the window and restore it. Similarly, if you right-click on the close button, the window disappears completely. You can restore it from the Windows Manager section of DM2.The application doesn't have an installer, so you can extract the files into a folder or copy them to your USB drive. Another impressive thing: the ZIP archive has only 135 KB. DM2 is open-source and it's free for noncommercial use.
Update: if you can't find the download link, here it is. Visit more often SourceForge and it won't seem that difficult to find your way.
Related:
The most powerful Windows application (Total Commander)
Convert your Windows into a beautiful Mac OSX