The best app, hands down. It has tips on what to do when confronted by law enforcement in various situations, and the ability to discretely record audio and video.
If you’ve ever had an encounter with the police that you wish you could record, here’s your ticket.
A word of caution: Some states and municipalities prohibit recording police and/or recording third parties. Know the law in your state and your city. However, and this is my non-legal opinion, it’s better to be safe than sorry when encountering possible police hostility/brutality.