Frankly, X a bit of a flop as a letter. It gets much more action as a symbol.
I. X is used to indicate multiplication in mathematics.
II. X is also used as a variable in mathematics.
III. X is used to indicate that something is incorrect or should be removed.
IV. There was a time in history where a lot of people were "signing with an X" to indicate a signature when they couldn't sign their name.
V. Xs and Os are used to indicate hugs and kisses in written form: XOXO
VI. Xs and Os indicate two different players in Tic-Tac-Toe.
VII. X is often used as shorthand for "Christ" in Christmas...surprisingly, not because it looks like a sideways cross. But Xmas has always made me think of X-rays and Superman or think that Santa's reindeer should have laser-shooting eyes.
VIII. X is used to indicate the female chromosome, where XX means female and XY means male. You can also have XXX chromosomes, which is Triple X syndrome, where you get an extra X chromosome. (Happens in 1 of every 1000 female births.)
IX. X is used in place of eyes to graphically indicate death (or fainting) in cartoons/comics/emoticons.
X. X indicates 10 in roman numerals.Now You!
How else is X used?