Below is a (hopefully) comprehensive list of all default Windows hotkeys. If (hopefully) you want to help add to this list:

1. State clearly what the hotkey does and, of course, what it is, in a readable format.
2. Mass posting pasted from websites is allowed, but be sure to state whether you've tested them or not. Also, a link to the site would be nice.
3. Feel free to improve this list as well, by testing untested hotkeys and posting your results.


Windows Hotkeys
Ctrl + Escape or Win: Display start menu.
Shift + F10 or AppsKey: Display context menu.
Win + E: Open Windows Explorer.
Win + R: Open Run dialog.
Win + M: Minimize all windows.
Win + Shift + M: Undo minimize all windows.
Win + D: Toggle minimize all windows.
Win + B (XP only): Activate system tray. Arrow keys select, Enter double-clicks, and AppsKey or Shift + F10 right-clicks.
Win + L (XP Only): Locks keyboard. Similar to Lock Workstation.
Win + F or F3: Open Find dialog. (All Files) F3 may not work in some applications which use F3 for their own find dialogs.
Win + Control + F: Open Find dialog. (Computers)
Win + U: Open Utility Manager.
Win + F1: Open Windows help.
Win + Pause: Open System Properties dialog.
Win + Tab: Cycle through taskbar buttons. Enter clicks, AppsKey or Shift + F10 right-clicks.

Win + Shift + Tab: Cycle through taskbar buttons in reverse.
Alt + Tab: Display CoolSwitch. More commonly known as the AltTab dialog.
Alt + Shift + Tab: Display CoolSwitch; go in reverse.
Alt + Escape: Send active window to the bottom of the z-order.
Alt + Shift + Escape: Activate the window at the bottom of the z-order.
Alt + F4: Close active window; or, if all windows are closed, open shutdown dialog.
Shift while a CD is loading: Bypass AutoPlay.
Ctrl + Alt + Delete or Ctrl + Alt + NumpadDel (Both NumLock states): Invoke the Task Manager or NT Security dialog.
Ctrl + Shift + Escape (XP Only): Invoke the task manager. On earlier OSes, acts like Ctrl + Escape.
Printscreen: Copy screenshot of current screen to clipboard.
Alt + Printscreen: Copy screenshot of current active window to clipboard.
Ctrl + Alt + Down Arrow: Invert screen. Untested on OSes other than XP.
Ctrl + Alt + Up Arrow: Undo inversion.


Generic

Ctrl + C or Ctrl + Insert: Copy.
Ctrl + X or Shift + Delete: Cut.
Ctrl + V or Shift + Insert: Paste/Move.
Ctrl + N: New... File, Tab, Entry, etc.
Ctrl + S: Save.
Ctrl + O: Open...
Ctrl + P: Print.
Ctrl + Z: Undo.
Ctrl + A: Select all.
Ctrl + F: Find...
Ctrl + F4: Close tab or child window.
F1: Open help.
F11: Toggle full screen mode.
Alt or F10: Activate menu bar.
Alt + Space: Display system menu. Same as clicking the icon on the titlebar.
Escape: Remove focus from current control/menu, or close dialog box.


Generic Navigation

Tab: Forward one item.
Shift + Tab: Backward one item.
Ctrl + Tab: Cycle through tabs/child windows.
Ctrl + Shift + Tab: Cycle backwards through tabs/child windows.
Enter: If a button's selected, click it, otherwise, click default button.
Space: Toggle items such as radio buttons or checkboxes.
Alt + (Letter): Activate item corresponding to (Letter). (Letter) is the underlined letter on the item's name.
Ctrl + Left: Move cursor to the beginning of previous word.
Ctrl + Right: Move cursor to the beginning of next word.
Ctrl + Up: Move cursor to beginning of previous paragraph. This and all subsequent Up/Down hotkeys in this section have only been known to work in RichEdit controls.
Ctrl + Down: Move cursor to beginning of next paragraph.
Shift + Left: Highlight one character to the left.
Shift + Right: Highlight one character to the right.
Shift + Up: Highlight from current cursor position, to one line up.
Shift + Down: Highlight from current cursor position, to one line down.
Ctrl + Shift + Left: Highlight to beginning of previous word.
Ctrl + Shift + Right: Highlight to beginning of next word.
Ctrl + Shift + Up: Highlight to beginning of previous paragraph.
Ctrl + Shift + Down: Highlight to beginning of next paragraph.
Home: Move cursor to top of a scrollable control.
End: Move cursor to bottom of a scrollable control.


Generic File Browser

Arrow Keys: Navigate.
Shift + Arrow Keys: Select multiple items.
Ctrl + Arrow Keys: Change focus without changing selection. "Focus" is the object that will run on Enter. Space toggles selection of the focused item.
(Letter): Select first found item that begins with (Letter).
BackSpace: Go up one level to the parent directory.
Alt + Left: Go back one folder.
Alt + Right: Go forward one folder.
Enter: Activate (Double-click) selected item(s).
Alt + Enter: View properties for selected item.
F2: Rename selected item(s).
F7: New folder.
Ctrl + NumpadPlus: In a Details view, resizes all columns to fit the longest item in each one.
Delete: Delete selected item(s).
Shift + Delete: Delete selected item(s); bypass Recycle Bin.
Ctrl while dragging item(s): Copy.
Ctrl + Shift while dragging item(s): Create shortcut(s).

In tree pane, if any:
Left: Collapse the current selection if expanded, or select the parent folder.
Right: Expand the current selection if collapsed, or select the first subfolder.
NumpadAsterisk: Expand currently selected directory and all subdirectories. No undo.
NumpadPlus: Expand currently selected directory.
NumpadMinus: Collapse currently selected directory.


Accessibility

Right Shift for eight seconds: Toggle FilterKeys on and off. FilterKeys must be enabled.
Left Alt + Left Shift + PrintScreen: Toggle High Contrast on and off. High Contrast must be enabled.
Left Alt + Left Shift + NumLock: Toggle MouseKeys on and off. MouseKeys must be enabled.
NumLock for five seconds: Toggle ToggleKeys on and off. ToggleKeys must be enabled.
Shift five times: Toggle StickyKeys on and off. StickyKeys must be enabled.


Microsoft Natural Keyboard with IntelliType Software Installed

Win + L: Log off Windows.
Win + P: Open Print Manager.
Win + C: Open control panel.
Win + V: Open clipboard.
Win + K: Open keyboard properties.
Win + I: Open mouse properties.
Win + A: Open Accessibility properties.
Win + Space: Displays the list of Microsoft IntelliType shortcut keys.
Win + S: Toggle CapsLock on and off.


Remote Desktop Connection Navigation

Ctrl + Alt + End: Open the NT Security dialog.
Alt + PageUp: Switch between programs.
Alt + PageDown: Switch between programs in reverse.
Alt + Insert: Cycle through the programs in most recently used order.
Alt + Home: Display start menu.
Ctrl + Alt + Break: Switch the client computer between a window and a full screen.
Alt + Delete: Display the Windows menu.
Ctrl + Alt + NumpadMinus: Place a snapshot of the entire client window area on the Terminal server clipboard and provide the same functionality as pressing Alt + PrintScreen on a local computer.
Ctrl + Alt + NumpadPlus: Place a snapshot of the active window in the client on the Terminal server clipboard and provide the same functionality as pressing PrintScreen on a local computer.


Mozilla Firefox 1.x

Ctrl + Tab or Ctrl + PageDown: Cycle through tabs.
Ctrl + Shift + Tab or Ctrl + PageUp: Cycle through tabs in reverse.
Ctrl + (1-9): Switch to tab corresponding to number.
Ctrl + N: New window.
Ctrl + T: New tab.
Ctrl + L or Alt + D or F6: Switch focus to location bar.
Ctrl + Enter: Open location in new tab.
Shift + Enter: Open location in new window.
Ctrl + K or Ctrl + E: Switch focus to search bar.
Ctrl + O: Open a local file.
Ctrl + W: Close tab, or window if there's only one tab open.
Ctrl + Shift + W: Close window.
Ctrl + S: Save page as a local file.
Ctrl + P: Print page.
Ctrl + F or F3: Open find toolbar.
Ctrl + G or F3: Find next...
Ctrl + Shift + G or Shift + F3: Find previous...
Ctrl + B or Ctrl + I: Open Bookmarks sidebar.
Ctrl + H: Open History sidebar.
Escape: Stop loading page.
Ctrl + R or F5: Reload current page.
Ctrl + Shift + R or Ctrl + F5: Reload current page; bypass cache.
Ctrl + U: View page source.
Ctrl + D: Bookmark current page.
Ctrl + NumpadPlus or Ctrl + Equals (+/=): Increase text size.
Ctrl + NumpadMinus or Ctrl + Minus: Decrease text size.
Ctrl + Numpad0 or Ctrl + 0: Set text size to default.
Alt + Left or Backspace: Back.
Alt + Right or Shift + Backspace: Forward.
Alt + Home: Open home page.
Ctrl + M: Open new message in integrated mail client.
Ctrl + J: Open Downloads dialog.
F6: Switch to next frame. You must have selected something on the page already, e.g. by use of Tab.
Shift + F6: Switch to previous frame.
Apostrophe ('): Find link as you type.
Slash (/): Find text as you type.


BBCode (This forum's posting)

While text is selected:
Alt + B: Format text in bold.
Alt + I: Format text in italics.
Alt + U: Format text as underlined.
Alt + Q: Format text as a quote.
Alt + C: Format text as code.
Alt + L: Format text as a list.
Alt + O: Format text as an ordered list.
Alt + P: Format url as a hotlinked image.
Alt + W: Format url as a hyperlink.


GMail

Must have "keyboard shortcuts" on in settings.
C: Compose new message.
Shift + C: Open new window to compose new message.
Slash (/): Switch focus to search box.
K: Switch focus to the next most recent email. Enter or "O" opens focused email.
J: Switch focus to the next oldest email.
N: Switch focus to the next message in the "conversation." Enter or "O" expands/collapses messages.
P: Switch focus to the previous message.
U: Takes you back to the inbox and checks for new mail.
Y: Various actions depending on current view:
Has no effect in "Sent" and "All Mail" views.

* Inbox: Archive email or message.
* Starred: Unstar email or message.
* Spam: Unmark as spam and move back to "Inbox."
* Trash: Move back to "Inbox."
* Any label: Remove the label.


X: "Check" an email. Various actions can be performed against all checked emails.
S: "Star" an email. Identical to the more familiar term, "flagging."
R: Reply to the email.
A: Reply to all recipients of the email.
F: Forward an email.
Shift + R: Reply to the email in a new window.
Shift + A: Reply to all recipients of the email in a new window.
Shift + F: Forward an email in a new window.
Shift + 1 (!): Mark an email as spam and remove it from the inbox.
G then I: Switch to "Inbox" view.
G then S: Switch to "Starred" view.
G then A: Switch to "All Mail" view.
G then C: Switch to "Contacts" view.
G then S: Switch to "Drafts" view.