This app was mentioned in 3 comments, with an average of 4.67 upvotes
For rooted phones:
Performance Profile
It allows you to increase/decrease the frequencies (in Mhz) of each app you open. I use it to run apps and games at the lowest frequency without allowing a noticeable drop in performance to save battery
For non-rooted:
My Class Schedule
Being a uni student, this allows me to keep track of upcoming lectures in which class, homeworks, holidays. It also auto silences your phone when class is in session, perfect for those absentminded students.
My class schedule app
>https://play.google.com/store/apps/details?id=de.rakuun.MyClassSchedule.free