This letter thanks an interviewer for the recommendation that resulted in your hiring. Frequently in large companies, you'll have very little direct contact with the person who hires you, so this is a great way to score some important points.
1234, Main Street
Boston, MA 02123
04 April, 2005
<Recipient Address Goes Here>
I wanted to write to express my sincere thanks for your time during our <interview/interviews>, and for the input that resulted in my hiring for the position of <position>. You made an often difficult process easy and pleasant, and I am thrilled to be working for <company name>.
Thank you again for all your courtesy and professionalism, and I look forward to working with you in the future.
encl: <List of enclosed items goes here>