Here are a few teachers and students have been using:
Number Pieces and Number Pieces Basic. The apps both contain sets of Base Ten Blocks (ones, tens, and hundreds), and tools for annotating. The apps can be used for modeling a variety of problems. This teacher is using the app to model decimal division.
Number Pieces: https://itunes.apple.com/us/app/number-pieces-by-math-learning/id605433778?mt=8
Number Pieces Basic:
The app can be used for learning about area, perimeter, fractions, the new circular Geoboard could help with learning about time, and much more.
The app is available for both iPad and iPhone: https://itunes.apple.com/us/app/geoboard-by-math-learning/id519896952?mt=8
The Math Learning Center has other great apps for learning math. Check them out too. Many of their apps are available for IOS, Windows phones and tablets, or via the web.