Useful Shortcut Keys In Ubuntu

Ctrl + A = Select all
Ctrl + C = Copy the highlighted content to clipboard
Ctrl + V = Paste the clipboard content
Ctrl + N = New (Create a new document, not in terminal)
Ctrl + O = Open a document
Ctrl + S = Save the current document
Ctrl + P = Print the current document
Ctrl + W = Close the close document
Ctrl + Q = Quit the current application

Keyboard shortcuts for GNOME desktop

Ctrl + Alt + F1 = Switch to the first virtual terminal
Ctrl + Alt + F2(F3)(F4)(F5)(F6) = Select the different virtual terminals
Ctrl + Alt + F7 = Restore back to the current terminal session with X
Ctrl + Alt + Backspace = Restart GNOME
Alt + Tab = Switch between open programs
Ctrl + Alt + L = Lock the screen.
Alt + F1 = opens the Applications menu
Alt + F2 = opens the Run Application dialog box.
Alt + F3 = opens the Deskbar Applet
Alt + F4 = closes the current window.
Alt + F5 = unmaximizes the current window.
Alt + F7 = move the current window
Alt + F8 = resizes the current window.
Alt + F9 = minimizes the current window.
Alt + F10 =  maximizes the current window.
Alt + Space = opens the window menu.
Ctrl + Alt + + = Switch to next X resolution
Ctrl + Alt + – = Switch to previous X resolution
Ctrl + Alt + Left/Right = move to the next/previous workspace

Keyboard shortcuts for Terminal

Ctrl + A = Move cursor to beginning of line
Ctrl + E = Move cursor to end of line
Ctrl + C = kills the current process.
Ctrl + Z = sends the current process to the background.
Ctrl + D = logs you out.
Ctrl + R = finds the last command matching the entered letters.
Enter a letter, followed by Tab + Tab = lists the available commands beginning with those letters.
Ctrl + U = deletes the current line.
Ctrl + K = deletes the command from the cursor right.
Ctrl + W = deletes the word before the cursor.
Ctrl + L = clears the terminal output
Shift + Ctrl + C = copy the highlighted command to the clipboard.
Shift + Ctrl + V (or Shift + Insert) = pastes the contents of the clipboard.
Alt + F = moves forward one word.
Alt + B = moves backward one word.
Arrow Up/Down = browse command history
Shift + PageUp / PageDown = Scroll terminal output

Keyboard shortcuts for Compiz

Alt + Tab = switch between open windows
Win + Tab = switch between open windows with Shift Switcher or Ring Switcher effect
Win + E = Expo, show all workspace
Ctrl + Alt + Down = Film Effect
Ctrl + Alt + Left mouse button = Rotate Desktop Cube
Alt + Shift + Up = Scale Windows
Ctrl + Alt + D = Show Desktop
Win + Left mouse button = take screenshot on selected area
Win + Mousewheel = Zoom In/Out
Alt + Mousewheel = Transparent Window
Alt + F8 = Resize Window
Alt + F7 = Move Window
Win + P = Add Helper
F9 = show widget layer
Shift + F9 = show water effects
Win + Shift + Left mouse button = Fire Effects
Win + Shift + C = Clear Fire Effects
Win + Left mouse button = Annotate: Draw
Win + 1 = Start annotation
Win + 3 = End annotation
Win + S = selects windows for grouping
Win + T = Group Windows together
Win + U = Ungroup Windows
Win + Left/Right = Flip Windows

Keyboard shortcut for Nautilus

Shift + Ctrl + N = Create New Folder
Ctrl + T = Delete selected file(s) to trash
Alt + ENTER = Show File/Folder Properties
Ctrl + 1 = Toggle View As Icons
Ctrl + 2 = Toggle View As List
Shift + Right = Open Directory (Only in List View)
Shift + Left = Close Directory (Only in List View)
Ctrl + S = Select Pattern
F2 = Rename File
Ctrl + A = Select all files and folders
Ctrl + W = Close Window
Ctrl + Shift + W = Close All Nautilus Windows
Ctrl + R = Reload Nautilus Window
Alt + Up = Open parent directory
Alt + Left = Back
Alt + Right = Forward
Alt + Home = go to Home folder
Ctrl + L = go to location bar
F9 = Show sidepane
Ctrl + H = Show Hidden Files
Ctrl + + = Zoom In
Ctrl + – = Zoom Out
Ctrl + 0 = Normal Size

(For those who want to configure your own keyboard shortcuts, you can do it at System->Preferences->Keyboard Shortcuts.)
Which one is your favorite keyboard shortcuts? Is there anymore shortcut that I have left out? Let me know in the comment and I will add it in.