Windows key shortcuts are my favorite because they never fail to impress. Winkey+E brings up an Explorer window, Winkey+R brings up the Run dialog, for example. Another favorite shortcut is using using ctrl-enter in IE. It appends "http://www." before and ".com" after whatever you type in the address bar. For example, if I type "Microsoft" and press ctrl-enter, it'll take me to "http://www.Microsoft.com". In the same vein of IE, holding down ctrl and pressing a number will take you to the corresponding tab ("ctrl-2" will take you to the second tab, "ctrl-8" will take you to tab 8, and 'ctrl-9" will take you to the last tab).
It's funny you mention backwards-compatibility.... Just the other day, I was working on an XP machine and subconsciously pressed "Winkey+Tab" to bring up Flip3D. What I found was that it cycles through items on the taskbar, much like Ctrl+Tab does in IE. Very cool!