Software Reference

Windows

Win+E: Opens Windows Explorer
Win+M: Minimises all windows
Ctrl+Shift+Esc: Opens Task Manager (or the much better Sysinternals Process Explorer, if you have it installed)

Web browsers

Alt+D: Moves to the address bar and select the current URL
Ctrl+Enter: Automatically adds a leading www. and trailing .com to what you have typed in the address bar, then navigate to it. So, for example, typing swref then pressing Ctrl+Enter will take you to http://www.swref.com.
F11: Toggles full-screen mode
Backspace: Goes back to the previous page

Microsoft Word

Ctrl+Alt+1: Applies the Heading 1 style to the current paragraph (the equivalent shortcut keys work for Heading 2 and Heading 3 as well)
Shift+F5: Jumps between recent edit points
Ctrl+Q and Ctrl+Space: Removes paragraph and character formatting respectively

Microsoft Excel

Ctrl+': Copies content from the cell(s) immediately above