Clarity Innovations has created a free Number Line App for the iPad. This app can be used to help students visualize the four major mathematical operations.
Options in the app allow you to change the tic marks for several different common increments, or you can even enter in your own custom numbering. There is a setting for tic mark spacing, and a setting to cover up the tic mark labels.
Once you adjust your settings, you can use the text tool to type in numeric expressions or equations.
Finally there is a jump tool where you can bring in your jumps, as many as you like, both forward and backwards for addition and subtraction. The jump interval can be manipulated to be as long or short as you wish, with the interval visible or not.
An annotation tool allows you to add free hand items for additional clarity.
Currently this app is limited to just positive numbers. Several people have posted comments requesting to have the number line extend into negative numbers. The app also does not provide a way to share your work, so if you want students to create images they would need to take a screen shot of their iPad using Home - ON/OFF sequence.
The Number Line App is a great resource for elementary students learning the concepts of addition, subtraction, multiplication, and division while also strengthening their number sense.