This app was mentioned in 2 comments, with an average of 2.00 upvotes
Actually I just noticed there's an android 'clapperboard' app.
https://play.google.com/store/apps/details?id=de.ueen.filmklappe&hl=en_CA
There is a nice free slate app missing from the android list:
https://play.google.com/store/apps/details?id=de.ueen.filmklappe