Few people, if any, send their interviewers "Thank You" letters - which is all the more reason that YOU should send one! After all, distinguishing yourself and making yourself memorable definitely put you at an advantage, especially if several other candidates are in the running with you.
Should you decide to send a "Thank You" letter to your interviewer, you'll have to skillfully walk the fine line between sincerely thanking your host, and opportunistically "marketing" your abilities.