Paper by Fifty Three is a free app that allows you to sketch, draw on images, take notes, make to do lists and more. Just create a space and start adding pages to it.
This is a great tool for students to draw out a process, show their work on math problems, draw pictures to illustrate a book or just take notes.
Another great feature - you and your students can use your school google account to sign in so you don't have to remember ANOTHER account name and password.
Finally, once you have a completed space, you can share it as a presentation and save it to your Google Drive or OneDrive.