Skitch is a free app available for both apple and android products.
It basically allows you to quickly annotate a photo taken from the device with text and arrows. The idea behind this is that after teaching students the names of various forces, the students are let loose within the classroom or school to take a photo of an item(s) and then draw force arrows and label it. They then check with the teacher that there are no errors.
I then plan to have students share their diagrams with other groups who can sketch them into their books.