ScreenChomp app on ipads. I've been overall pleased with the way that my students have produced these screencasts and think that it is providing some powerful metacognition.
Here is an example of a 7th grader's screencast on basic geometric figures:
Here is an example of an 8th grader's screencast on using proportions to solve:
-Having students create these screencasts have been a valuable way for the student to show me that they know a particular skill, and, its been fun for students!
-From a math standpoint, ScreenChomp is my favorite free app, and I like that you can scroll down on the whiteboard for more space (I haven't seen any other that give that extra space).
-For my intervention students, who generally are in the 20th-50th percentile on state tests, articulating their thinking is challenging, but so good when they're able to make these mathematical connections!
-As of now, I've used this only for formative assessment, but have a goal of using screencasting for a summative assessment as well.