Lubuntu LXDE Desktop Keyboard Shortcuts
Alt + Tab: Toggle between currently open windows.
Alt + Shift + Tab: Toggle between currently open windows in reverse order.
Alt + F4: Closes current window.
Ctrl + Alt + T: Launch Terminal.

Default LXDE/Openbox bindings
Action Binding
Go to desktop (direction): ctrl + alt + arrow
Go to desktop (number): super + f1/f2/f3/f4
Send to desktop: shift + alt + arrow
Toggle show desktop: super + d (super is the Start key on many keyboards)
Close window: alt + f4
Hide window: alt + esc
Window menu: alt + space
Next window: alt + tab
Previous window: alt + shift + tab
Go to window: (direction) super + shift + arrow
Toggle fullscreen: f11
Window menu: alt + spacebar (useful for off-screen Skype windows in Lubuntu 13.04 Raring Ringtail)

LXDE specific bindings

Action                  Binding           Alt. binding
Window list (sticky):   ctrl + alt + tab (not in 13.04)
PCMan File Manager:     super + e         ctrl + alt + d
LXPanel Run:            super + r         alt + f2
LXPanel Menu:           alt + f1          ctrl + esc (apparently not in Lubuntu 13.04)
LXTask (task manager):  ctrl + alt + del
Open terminal:          ctrl + alt + t
Lock screen:            ctrl + alt + l

The absence of control-escape LxPanel menu functionality can be problematic if one's mouse is dead in Lubuntu 13.04. The only fall back at the moment seems to be ctrl-alt-t and then run programs from the command line. This is likely an issue between OpenBox and LXDE that may have appeared circa 11.07.

Lubuntu specific bindings
Action: Binding
Take a screenshot: alt + print screen

The following are Ubuntu Unity shortcuts, some of which work for Lubuntu as well.

Ubuntu Unity Desktop Keyboard Shortcuts
Alt + Tab: Toggle between currently open windows.
Alt + Shift + Tab: Toggle between currently open windows in reverse order.
Alt + F1: Put keyboard focus on the Launcher, use arrow keys to navigate, Enter launch.
Alt + F2: Opens dash in special mode to run any commands.
Alt + F4: Closes current window.
Alt + F7: Moves the current window(both keyboard and mouse can be used).
Alt + F9: Minimize current window.
Alt + F10: Toggle between Maximize/Unmaximize current window.
F10: Open the first menu on top panel, use arrows keys to browse across the menus
Press and release super Key (Windows Key): Opens dash.
Hold super Key: Invokes Launcher.
Hold super Key and hit 1, 2, 3 etc.: Open an Application from Launcher.
super + A: Opens up application window from launcher.
super + F: Opens up files and folders window from launcher.
super + W: Spread mode, zoom out on all windows in all workspaces.
super + D: Minimize all windows(acts as Show Desktop). Hitting it again restores them.
super + T: Opens trash can.
super + S: Expo mode, zooms out on all the workspaces and let's you manage windows.

Ctrl + Alt + T: Launch Terminal.
Ctrl + Alt + L: Lock Screen.
Ctrl + Alt + Left/Right/Up/Down: Move to new workspace.
Ctrl + Alt + Shift + Left/Right/Up/Down: Place window to a new workspace.

Ctrl-Alt-Numpad 1: Place window in the bottom left corner of the screen.
Ctrl-Alt-Numpad 2: Place window in the bottom half of the screen.
Ctrl-Alt-Numpad 3: Place window in the bottom right corner of the screen.
Ctrl-Alt-Numpad 4: Place window on the left side of the scren.
Ctrl-Alt-Numpad 5: Center/Maximize the window in the middle of the screen.
Ctrl-Alt-Numpad 6: Place window on the right side of the screen.
Ctrl-Alt-Numpad 7: Place window in top left corner of screen.
Ctrl-Alt-Numpad 8: Place window in top half of screen.
Ctrl-Alt-Numpad 9: Place window in top right corner of screen.
Ctrl-Alt-Numpad 0: Maximize window.

There are also some tricks you can do with your mouse, such as:
Maximizing: Dragging a window to the top panel will maximize it.
Middle click on Maximize: Maximize Window Vertically.
Right click on Maximize: Maximize Window Horizontally (unless mouse buttons set left handed, then reverse)
Tiling: Dragging a Window to the left/right border will auto tile it
Restoring: Dragging the top panel down on a maximized window will restore it to its original size.
Middle click on an application's launcher icon: Open a new instance of the application in a new window. If the application isn't running it will just open it normally.
Middle click on the top panel (but not the menu): Send the current window behind all other windows.
Middle click on Maximize: Maximize Window Vertically.
Right click on Maximize: Maximize Window Horizontally.