I've heard that math teachers are really loving Poll Everywhere (for polls) and Desmos (for graphing). Plus Illuminations has translated a few of there online apps into mobile apps: Okta's Rescue and Concentration.
What about you? Any favorite apps for smartphones or tablets that you're using in your math class?
Here's one more.
Teacher’s Assistant Pro is a great app for tracking student behavior, participation, and achievements. Anyone else using it?