Cleanflight

IMPORTANT NOTICE: Support for STM32F1 based flight controllers will be removed in late 2017, this includes NAZE, CC3D (original) and CJMCU like flight controllers.

Cleanflight is flight controller software for multi-rotor and fixed wings. The Cleanflight project, and related projects such as Betaflight and iNav are used on the majority of flight controllers used around the world. There is no other software used on as many flight-controllers!

If you're looking for experimental new features and don't mind doing your homework, checkout the betaflight fork.
If you're looking for advanced navigation features then check out the iNav fork.
All other users should use Cleanflight.

Features:

Awesome flight performance as trusted by the majority of Acrobatic and Racing Drone pilots.
Support for modern STM32 based processors F1/F3/F4/F7.
Support for modern accelerometer/gyro/barometer/compass sensors.
Support for modern ESC technologies DSHOT/ONESHOT and legacy PWM.
Support for Multi-color RGB LED strip support.
Advanced on-board telemetry logging (Blackbox).
Wide support of receivers (SBus/iBus/SumD/SumH/PPM/PWM/CRSF/JetiExBus)
Wide support of telemetry protocols (FrSky/SmartPort/S.Port/HoTT/iBus/LTM/MavLink/CRSF/SRXL).
Built-in OSD support & configuration without needing third-party OSD software/firmware/comm devices.
Support for external OSD slave systems.
VTX support (RTC6705/Unify Pro(SmartAudio)/IRC Tramp/etc).
and MUCH, MUCH more.

Installation & Documentation

Cleanflight documentation - https://github.com/cleanflight/cleanflight/tree/master/docs
Betaflight Wiki - https://github.com/betaflight/betaflight/wiki

Support

Your first place for support are the Cleanflight forums on RCGroups

The Github issue tracker is NOT for end-user support.

IRC Support and Developers Channel

There's a dedicated Cleanflight IRC channel on the Freenode IRC network. Many users and some of the developers frequent there, and it is a helpful and friendly community - but there are two important things to keep in mind: First and most importantly, please go ahead and ask if you have questions, but make sure you wait around long enough for a reply. Next, sometimes people are out flying, asleep or at work and can't answer immediately, even though they are present in the channel. This is how IRC works: Many people stay logged in, even though they are not actively participating in the discussion all the time. Have a seat, grab a drink and hang around if it's a quiet time of day.

irc://irc.freenode.net/#cleanflight

If you are using Windows and don't have an IRC client installed, take a look at HydraIRC.

There's a dedicated Slack chat channel for betaflight here:

http://www.betaflight.tk/

Etiquette: Don't ask to ask and please wait around long enough for a reply - sometimes people are out flying, asleep or at work and can't answer immediately.