The following post is taken from Ubuntu Unleashed, one of the cooler sites covering all things Ubuntu. I am reposting it, to remind us of the cool short-cuts available on the Free Gnome Desktop.
Alt-Ctrl-L Locks the screen.
Alt-F1 opens the Applications menu, then use the arrow keys to navigate the submenus.
Alt-F2 opens the Run Application dialog box.
Alt-F3 opens the Deskbar Applet (F3 opens the search bar at the bottom of the window).
Alt-F4 closes the current window.
Alt-F5 unmaximizes the current window (if it’s maximized, of course).
Alt-F7, followed by arrow keys or mouse movement, adjusts the current window’s position.
Alt-F8 resizes the current window.
Alt-F9 minimizes the current window.
Alt-F10 maximizes the current window. Alt-spacebar opens the window menu.
Alt-Tab moves between open windows.
Ctrl-Alt-Tab moves between open panels on the desktop.
Ctrl-W closes the current window.
Ctrl-Q closes the current application. Here are some keyboard shortcuts for working in the Terminal window:
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.
Tab followed by entered letters lists the available commands beginning with those letters.
Ctrl-U deletes the current line.
Ctrl-K deletes from the cursor right.
Ctrl-W deletes the word before the cursor.
Ctrl-L clears the terminal output.
Shift-Insert pastes the contents of the clipboard.
Alt-F moves forward one word.
Alt-B moves backward one word.
Ctrl-Alt-arrow to move between desktops.
You can use Ctrl-Alt-F1, Ctrl-Alt-F2, Ctrl-Alt-F3, Ctrl-Alt-F4, Ctrl-Alt-F5, Ctrl-Alt-F6 to access the consoles and Ctrl-Alt-F7 to come back to the desktop.
and if you have compiz-fusion, then you can use the Windows-key (WK):
WK+Tab to change between the windows
WK+E to select a virtual desktop
WK+S to make transparent the current window
Here is another useful combination:
Ctrl+Alt++ = Switch to next X resolution
Ctrl+Alt+- = Switch to previous X resolution
Alt + D for select current browser window url
ALT+CTL+BACKSPACE closes desktop and lets you login again
ALT-scroll wheel to adjust transparency in current window