I made a simple Hydration Calculator. http://joshuacronemeyer.github.com/Flour-and-Water/  I've tried to use a few other online hydration calculators and there are so many textboxes that I don't know what to do with them all! This one has NO text fields. Instead, each colored box represents one or more variables: flour, water, starter. Just drag the box bigger or smaller to change the quantity and everything is calculated on the fly. I love to make sourdough and I'm always playing with my recipes, or finding myself with too little or much of one ingredient or another. I think this calculator makes it easy to play with a recipe and tailor it to your own needs. I'm no professional baker so it might not seem suitable for the pros, but I've checked it against a handful of my favorite recipes and found it gives accurate results. It is open source as well so if you are inclined to dabble in that sort of thing you can click the 'fork me on github' banner to get your own copy of the source code (Maedi I'm talking to you) ;).