How I Use It
My school has an account for Go Math's Think Central and allows us access to the Primary Tools (though I have found it for free through a Google search). Primary Tools has a ton of options for showing math manipulatives to visually demonstrate everything from graphs to 3-digit addition and subtraction.
I use these manipulatives on the Smart Board for whole class demonstration of concepts with 2nd graders. Some of the functions (adding and subtracting and money amounts) allow for feedback. My students love clicking the 'check' button to see if they were correct. It works well with the Smart Board to make it more hands on, almost as if you were really handling the manipulatives. I believe it is in Flash, so iPad and iPhone usage may not be as easy.