cleanflight.com

Cleanflight can be used on multirotor aircraft and fixed-wing aircraft, it supports a variety for shapes and motor counts, not limited to quadcopters, hexacopters, octocopters, tricopters and planes. Cleanflight is Open-Source flight controller software which is 32-bit version of the original 8-bit MultiWii code.

FEATURES
Awesome flight performance
Most people that have tried other flight controller software are impressed how stable and responsive aircraft running cleanflight fly.

Open-Source
Source code is on github, you're free to modify it and submit improvements. GPL Licensed. Issue tracker. Pull requests.

Modern 32-bit hardware support
When 32-bit equipped flight controllers are so cheap, why hold yourself back with old 8-bit hardware and software? Cleanflight leverages the capabilities of modern embedded processors to give you the best control.

Community driven
Cleanflight maintainers actively seek and listen to user feedback from users. Friendly community. Active developers. IRC Channel.

Modern development principles
As software grows over time it often becomes harder to maintain and fragile; Cleanflight attempts to bring modern software development practices to the table. We like and encourage understandable, decoupled, well-tested and self-explanatory code.

Broad hardware support
Cleanflight currently runs on 8 different boards and supports enough sensors and CPUs for you to adapt it to you own custom flight controller board, even more off-the-shelf hardware support is planned.