This is my #1 must-have as a teacher without a SMART Board. My first year teaching, every classroom had SMART Boards, and some teachers didn't even know how to use them! At my school, we don't have SMART Boards, but that hasn't stopped me from finding a way to teach in a similar manner. To be honest, I prefer Doceri Desktop to my SMART Board and would back that up any day.
Doceri Desktop is a remote desktop app for tablets but it also has notebook capabilities which allows me to put up a dry erase board, lined paper, grid paper, maps, etc and annotate from my iPad. Furthermore, I am able to pass the iPad around to students and they can show their work in math on the projector from their seats. It is an incredible tool.