WSCS Tech

This is what I use to make almost all my math notes.  You can use it as a interactive whiteboard starting with a blank canvas or with slides and then write on them.  You can record to make it a video or export it as other formats.  I LOVE this app.  $15 and would pay it again in a heartbeat.  

This video quickly explains the main features of the program.

Some free options are listed below (Show Me & Educreations) but free ones like these often are quite limited.  They might be a nice intro into what these can do, though.