ex () - x-height of container font size (see note below).
ch () - Width of "0" in container font size.
rem () - Width of "m" in root (<html>) font size.
vw () - 1% of width of window.
vh () - 1% of height of window.
vmin () - 1% of min(width, height) of window.
vmax () - 1% of max(width, height) of window.
% () - 1% of container width.
Note: an "em" is traditionally the with of the letter "m", but in CSS it's defined as equal to the font size. For example, in this font at 14px, an "m" is 13.28px wide, but one "em" is 14px. Credit to Julia Evans for pointing this out.