Mirrycle MTB mirror. I can't believe I rode on the street without one for so many years. Best $15 I ever spent on my bike.
Edit: Also, the AAA-powered mp3 player and AA-powered speaker on my handlebar (total = less than $35 about five years ago). These have added immeasurable joy to my commutes and to my overnight rides.
Last edited by Papa Tom; 07-30-16 at 08:33 PM.