We mentioned Michael Ruhlman's Ratio iPhone app back in December, but it looks like Ruhlman just released an updated version, available in the iTunes Store for $4.99 (he says they're working on a Droid version). The application includes 32 critical ratios and will calculate the amount of each ingredient you need based on what you already have or want to make. It also includes basic recipes and variations and allows you to save your own recipes and notes. Ruhlman demonstrates the application in the video above.
Related: Michael Ruhlman's New Book: 'Ratio'