If you haven't signed up for their Free App Friday e-newsletter yet, I would recommend doing so! Lots of great stuff!
Another free app that can be used in your classroom for Halloween fun is Doodle Buddy. Using the stamps function, you could ask kids to create and complete equations with fun Halloween themed icons. You could also have them arrange a group of objects and circle what is more or less. It could also be a fun way to illustrate a spooky Halloween-themed story. Lots of creativity with Doodle Buddy!