ALT+TAB: This is now the equivalent of knowing what the "enter" key does. You should certainly have a good handle on it if you are in front of a computer for more than 5 minutes on any given day.
WIN+TAB: On Aero themes, it's ALT+TAB on steroids
ALT+ESC: Quickly minimize what you're looking at. Great for when the boss sneaks up on you while you're browsing gonewild or something. Bonus is that it doesn't give the telltale "blip" on the screen of ALT+TAB that any boss worth his salt will recognize.
CTRL+X, C and V: Cut, Copy, Paste...respectively.
CTRL+F, or H: Find, Replace...respectively.
CTRL+A: Select All
SHIFT+Arrow Keys: Select text
CTRL+Arrow Keys: Move across words in text
CTRL+SHIFT+Arrow Keys: Select words/lines/paragraphs of text.
CTRL+Backspace: Deletes previous word/character block
CTRL+Delete: Deletes next word/character block
CTRL+SHIFT+ESC: Windows Task Manager. No more ALT+CTRL+DELETE!
WIN+L: Lock yo shit
WIN+M or WIN+D: Minimize all the things.
CTRL+B: Toggle Bold formatting (works in most word processors or other text-formatting editors)
CTRL+I: Toggle Italic formatting
CTRL+U: Toggle Underline
WIN+ R: Open "Run" Prompt


WIN+Arrow keys: Shift position within current monitor
WIN+SHIFT+Arrow keys: Move window to different monitor.

Windows Explorer

Note: many of these work the same in most browsers where applicable

WIN+E: Open new instance of Windows Explorer at the My Computer location
ALT+D: Move cursor to the location/url bar.
CTRL+W: Close tab or browser window.
CTRL+N: When already in Explorer, this opens a new instance of explorer at your current location
SHIFT+DELETE: Deletes a file, bypassing the recycle bin.
CTRL+SHIFT+N: New folder
F2: Rename file. Tab from this to continue down the row with renaming.

Browsing the intertubes...