When you go on a job interview, it is (now?) customary to send a thank-you letter to the interviewer. Astute observers have pointed out that a job interview is not some act of benevolence to be grateful for so much as an opportunity for both parties to examine a potential business transaction in further depth.
a) It's a buyer's market, so you have to do everything you can to ingratiate yourselves to potential employers ATM.
b) It's good PR to send them the thank-you letter because it keeps *them* thinking about *you*, which increases the likelihood that you will receive the offer if it comes down to a toss-up.
What do you put in your thank-you letters?