Some really interesting things here.
One of my favorites is a plug in that allows .pdf files to viewed in Explorer if you are using FoxIt reader. Only for Vista at the moment but they are working on other versions.

Download Free Windows Vista Software from Microsoft
http://bhandler.spaces.live.com/blog/cns!70F64BC910C9F7F3!1844.entry

Download Free Office 2007 Software from Microsoft
http://bhandler.spaces.live.com/blog/cns!70F64BC910C9F7F3!1822.entry

I ran across this little gem at Lifehacker http://www.lifehacker.com/ today.

It allows your mouse to scroll in *any* window or pane your mouse is in.
Not being able to do that has been an annoyance of mine for a long time.
I'm happy to report it works great, and the tray icon is cute (not a requirement but fun anyway).

http://www.code-scout.net/katmouse/
donationware

from the site:
"The prime purpose of the KatMouse utility is to enhance the functionality of mice with a scroll wheel, offering 'universal' scrolling: moving the mouse wheel will scroll the window directly beneath the mouse cursor (not the one with the keyboard focus, which is default on Windows OSes). This is a major increase in the usefullness of the mouse wheel.

Another feature involves the wheel button. Since the wheel button is not consistently used in Windows, KatMouse can use it for a kind of task switching: with a click of the wheel button you can push a window to the buttom of the stack of windows that is your desktop, making a recovered window the active window."