iPad app and as a Chrome app. It can also be used directly in any updated web browser. The browser-based version can be found here.
Number Rack provides a set of virtual number beads that are grouped into sets of five red and five white beads. Number Rack on the iPad allows students to have up to 10 rows of beads. Number Rack on the web provides up to five rows of beads. You can obscure some of the beads to model subtraction and addition with the virtual beads. Number Rack for iPads is available here and it is available here for the web.
Applications for Education
Neither of these apps offer anything ground-breaking. If you're looking for mobile versions of classic elementary school math activities, these apps are worth a look.