Thank You
Thank you letters are often an untapped resource for selling yourself to a potential employer.
You did your job at the interview, you asked questions, and took notes. After getting home, you synthesized what you learned into a further evolution of your argument for why they should hire you. Don't fail to continue this argument in your Thank You letter! There are things you were told about their internal plans for growth, problems they are focused on, WAYS YOU COULD HELP THEM!
Your ability to hear, understand, and integrate what you were told at the interview into more ideas for why YOU are the right candidate for the Job is evidence in action! Think before you just say Thank You.