XnView Software

Visual Task Tips
A lightweight shell enhancement utility.
It provides thumbnail preview image for each task in the Windows Taskbar, as seen in the upcoming Microsoft Windows Vista.

Note Windows XP is required to run Visual Task Tips.

Download: http://www.visualtasktips.com/download.php
Homepage: http://www.visualtasktips.com/

You may also want to give a try to: Visual Tooltip.
It is more flexible (and more complex... ) and offers a Dockbar as well. It's seems still quite efficient RAM/CPU wise.

I tried them... and never actually use them...

Thanks, Olivier!
I must admit I'm getting bored with it too.