Compiz (the default effects engine that can be turned on) has had a Window Previews plugin since well-before Windows 7 was around. It just provides a preview for a window. It doesn't stack them and it only works for windows that aren't minimised (although I think there are some hacks to get around that).
To enable it, turn on compiz:
- Right click the desktop
- Click Change Desktop Background
- Select the Visual Effects tab
- Use Normal or Extra
Extra may turn this on by default (I don't want to test it as I like my current compiz settings). Test it by hovering over a running program.
If it doesn't you want the CompizConfig Settings Manager (
sudo apt-get install compizconfig-settings-manager). Once installed, load it from the System -> Preferences menu.
Use the filter box to search for "preview" and turn the option on by checking the checkbox. You'll end up with something like this: