Here are some handy little add-ons by Microsoft.
*URL REMOVED*
I am a very big fan of the alt-tab replacement and the power calculator. The alt-tab replacement allows you to see a preview of the windows as you al-tab through them so you can choose the right one. The Calculator performs all sorts of conversions.
I also like the open command prompt widow here function. It allows you to right-click and folder and select command prompt and have the command window open up directly the that folders path.
For graphics junkies, the image resizor allows for easy image resizing with just a right click.
Many more things are available, but not all are useful to me. Some of you may find the other stuff useful.