Apps For Arduino is a website that specializes in creating software applications that work specifically with your hardware projects. You download and install the app on your Mac or iOS device and it communicates with the Arduino via a serial connection. While not all the apps are free, those which aren’t can be “purchased” as Tweetware (i.e., you will have to perform one (or more) micro-actions on the honor system to “pay” for the app.) The corresponding Arduino sketches/libraries are provided for free download.
Apps currently available provide graphical representations of buttons or an analog style meter (pictured above) which respond to the Arduino data. There is also Matatino, an open source Cocoa framework for direct serial communications with the Arduino, and Wijourno, which facilitates connections between your Mac and your iOS devices using Bonjour!