I have recently released 'Mix Blender' for the iPhone. It computes Nitrox and Trimix blends, producing simple, easy to read 'recipes'. It also will compute top-ups. It has a bit of a unique feature in that it will not only do 'best mixes' based on a desired PPO2 and END, but also will allow the user to dial-a-depth and see the recommended UTD and GUE standard gas for that depth. The software works in both metric and imperial.
Several updates are on the way, including fill costing, fudge factors, Z factors, real gas law calculations and multiple languages. I will also be adding some cascade management features at some point. The application can be had here: