X-ing
X-ref
Luxury
Generation X
X-ray
Xmas
exam
fax
xenophobia
Xerox
Faux
X Rated
X's and O's
Xian
X is the unknown
Xmit
X-files
Malcom X
3 x 2
X, IX, VIII

But if you've lost something you treasure,
Then X marks the spot.