Common:
* X-ray
* Xenon
* Xerox
Less common:
* Xylophone
* Xanthous (a person with yellow hair)
* Xylem (plant tissue)
* Xiphoid process (part of the sternum)
* Xerophyte (a plant adapted to dry environments)
* Xanthium (a genus of plants, commonly called cocklebur)
* Xylophagous (insect that eats wood)
Let me know if you'd like more examples!