Windows/Mac/Linux: Brewtarget is a free, open-source application designed to help homebrew enthusiasts create and organize their beer recipes.

From the project homepage:

I was fed up with the lack of good open source beer software, so I thought I would make a small application that would do all the calculations for me.

The ultimate goal of the project is to be able to take a set of user-given constraints and immediately formulate a recipe.

If Brewtarget turns out not to be your pint of beer (har har), brewsta is another free, cross-platform, open-source alternative worth a look. This editor's definitely going to forward these apps onto his homebrewing dad, but if we've got any other experienced homebrewers out there who've got their own homebrew hacks and helpers, let's hear about it in the comments.

Brewtarget is a free, open-source application for Windows, Mac OS X, and Linux.