Skitch IPad app. I use it all the time in school, and find it very easy for my students to use. In fact, there have been times when they have been able to show me some features that I was unaware of.
My most recent experience with the app was while we were learning about the different types of angles. To assess their new vocabulary and understanding of the different angles, I sent them out of the room in small groups to search for real examples of acute, right, obtuse and reflex angles. Each student was responsible for a different angle.
First, they would take a picture of an angle within the app. Skitch then allows the photographer to chose whether or not they want to use the photo. This is a great feature because it avoids a huge confusing stockpile of blurry images with an errant thumb halfway covering the lens. Next, the students will draw the angle using the arrow tool, tracing over the part of the image that represents their angle. It is important that the student starts drawing from the vertex of the angle, otherwise the arrows will appear to be pointing towards the vertex. Next, using the text tool, they would name the angle. Finally, I printed each photo to display on our hallway bulletin board.
My students thoroughly enjoyed this activity, and were very creative with what they chose to represent their respective angles. Here are just a few examples of what they were able to produce.