VLC - video playback app that can be added to touchscreen Chromebook devices
Pocketcast - If you're a podcast-liking person, this keeps track of all your podcasts in one place. While it does cost 3.99, that is a one time fee and it can be accessed on any device with your that payment.
Screencastify - records your screen and your voice and/or video along with it. This can be used for students to talk through a presentation or for teachers to create a flipped classroom.
Habitlab - This site is one for those students who are frequently off task. Maybe they spend too much time on Twitter or Facebook. If this is the case, this addition will attempt to re-train people and get them to spend less time on websites that are hindering productivity.