Mathlearningcenter.org is a site that contains numerous free resources to try with your students. There is a web version available as well as apps available to add in Chrome and iOS. The apps/sites are extremely user friendly. What I really like is that you can share the work created directly in the site as a direct link or download an image to share. This is extremely helpful for students that may have missed a lesson or would like to see the work again.
Also, when selecting the app make sure to check out the tutorial as well as select the i to see helpful How To information.
Embedded below is a 2 minute and 15 second YouTube tutorial (I show the math clock app).