This open source calculator can be used for the calculation of hydroponic nutrient recipes. You can give hydrobuddy a target set of concentrations for common nutrients and a selection of substances to use and hydrobuddy will tell you how much to use of each one to get as close as possible to your targets. Hydrobuddy is a powerful tool for the preparation of hydroponic nutrient solutions.
If you want to build hydrobuddy from source you will need to download Lazarus >= 1.6.4. Previous complicated requirements have been removed and the program should now build just by opening the project file in Lazarus and compiling it. For more information about hydrobuddy and hydroponics please visit my blog.
If you want binaries for Windows/Linux you can find them here.