The sternocleidomastoid muscle. It's a mouthful, but it's a favourite of mine just because of the name and the lovely stretch that you can do to help release any tension in this muscle.
The name of this muscle always made the most sense in my anatomy class as the name itself tells us where it originates (sternum and clavicle) and where it attaches (mastoid process behind your ear). This is always an anatomy test question, by the way :-)
The sternocleidomastoid functions to help rotate the head to the side that is contracting, flexes the neck laterally, and when both sides are working together it will flex the neck forward. Tightness can occur in this muscle by spending long periods looking down at our keyboards or our phones. Over time this muscle can shorten causing poor posture with the alignment of our head, neck and shoulders.
This is when the lovely 'stargazer' stretch comes in handy. Just anchor one hand on the sternum and clavicle, turn your head to the diagonal and look up to the stars. Play with the angle of your head and you will feel a pull in the neck that might refer around the eye. Hold the stretch for approximately 30 seconds and don't forget to stretch the other side.