The Stockfish engine was developed by Tord Romstad, Marco Costalba, and Joona Kiiski. It is now being developed and maintained by the Stockfish community.
Stockfish for Mac was built by Daylen Yang. Stockfish for iOS was built by Tord Romstad.
The Stockfish project started with the open source Glaurung engine, authored by Tord Romstad. In November 2008, Marco Costalba forked the Glaurung 2.1 code and introduced Stockfish 1.0. Tord and Joona Kiiski joined the Stockfish project and the Glaurung project slowly faded away. Meanwhile, Stockfish quickly rose to become the strongest open source chess engine, with frequent updates every few months. Today, it remains one of the strongest engines in the world.
Stockfish is free, and distributed under the GNU General Public License Version 3 (GPLv3). Essentially, this means that you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your web site, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.
The only real limitation is that whenever you distribute Stockfish in some way, you must always include the full source code, or a pointer to where the source code can be found. If you make any changes to the source code, these changes must also be made available under the GPL.
For full details, read the GPL.