Websites
Calm (free trial only)
mindful (low-cost subscription)
Mood Tracker (free and upgraded versions available)
Headspace (low-cost subscription)
Mobile Apps
Insight Timer (free)
Simple Habit (in-app purchases)
Breathwrk (in-app purchases)
Smiling Mind (free)
Rootd (in-app purchases)
Mindshift (Free)
Books
Practicing Mindfulness by Matthew Sockolov
Mindfulness for Teens in 10 Minutes a Day by Jennie Marie Battistin
5-Minute Mindfulness Meditations for Teens by Nichole Libin