Turn Caps Lock into Ctrl

Add this line to your ~/.profile file and log out and back in:

setxkbmap -option "shift:both_capslock,caps:ctrl_modifier"

Caps lock will act as Ctrl, and if you press both Shift keys at the same time, it will toggle caps lock.

Bonus: Swap Home/End with Page Up/Down

If you have a keyboard with page up/down keys next to your arrow keys but find that you never use them, you can swap them with home and end so it is easier to quickly jump to the beginning and end of a line when editing text.

Just edit this file with sudo nano /usr/share/X11/xkb/symbols/pc and change the following section:

    ...
    key <HOME> {    [  Home         ]   };
    key <PGUP> {    [  Prior        ]   };
    key <DELE> {    [  Delete       ]   };
    key  <END> {    [  End          ]   };
    key <PGDN> {    [  Next         ]   };
    ...

to look like this:

    ...
    key <HOME> {    [  Prior        ]   };
    key <PGUP> {    [  Home         ]   };
    key <DELE> {    [  Delete       ]   };
    key  <END> {    [  Next         ]   };
    key <PGDN> {    [  End          ]   };
    ...

Congrats! You have boosted your productivity by 9000%!