HR question: Importance of the follow-up?

Someone recently broached the question of a follow-up contact made after an interview. HR Professionals or anyone who has experienced the situation: How important is the follow-up email/phone call/letter from the applicant? In some cases, does it make the applicant stand out, seem desperate, or seem obnoxious? Do other reasonable applicants get bumped for the ones who follow-up? When is the best time to follow-up and how?