Every time you think of a word, tiny muscle movements in your larynx echo your thought. You "say" the word subvocally, imperceptibly, unconsciously.
Now researchers have developed a wheelchair steered by this natural phenomenon known as subvocalization. For all practical purposes, we now have a simple way to steer things using thoughts.
Below, a video explaining how it works.
The system isn't perfect for severely paralyzed patients who can't move facial muscles, but it's still pretty darn effective.
See also: steering with your tongue.