I find that I waste way too much paper when I have students do their first few problems on paper. Most of my students throw the paper out rather than use it as an example for later, so I have them use an app like doodle buddy or SAS gloss. Doodle buddy is more of an art program where students can use their fingers to try the problem. It allows them to choose the background, the ink, insert text, etc. I usually walk around the class checking their progress, answering questions, etc. if they have done the problem correctly, I draw a smiley face, if not, they get a frowny face. They can save the image and email it if desired. The other program offers a bit more.
Gloss offers the drawing surface but it also allows you to choose graph paper, wide ruled or college ruled paper as a background. This program also allows you to add shapes, write or insert text and is easier to use in a math class room. I am going to install this program in a day or two and try it out. I am thinking it might offer me a way to increase the amount of writing I have them do in class. I will keep you posted on how it goes.