Made a browser app which used androids web view, added a few buttons and gave it to a friend and forgot about it. About a year later he messaged me he still used that dumb app. Lol

Also made a notes app for Android that just had one text view with only options to edit text and save it. Used it for years for pasting random stuff, and gave it to a twitter friend and forgot about it. Connected with that friend years later and he said he used that app as well :)