I've been working on Yet Another RepRap Firmware (Yarf) and I've just released version 0.1.
Yarf's key features include:
* True linear acceleration and deceleration
* A lookahead algorithm (based on grbl)
* Includes Matt Roberts' advance algorithm
* PID temperature control
* Runs on common 8-bit MCU's such as the ATmega644p and ATmega1284p
* Direct support for thermistor tables generated by tltgen
* Fast enough to drive four 1.8 degree stepper motors with 1/16th microstepping
* Extensive source code documentation
Compared to other recent firmwares, Yarf does not add any revolutionary new features. The extensive documentation is currently its greatest merit. This documentation allows developers to easily get acquainted with the source code. Care has been taken to structure the code into different components, each of which encapsulates a different responsibility of the firmware.
Yarf is a good firmware to take a look at for anyone interested in the 'inner workings' of a RepRap firmware. Moreover, it is a good base to start from if you want to try out a new firmware feature but do not want to start from scratch.
For more info, you can check out the Yarf wiki page and my blog (see my signature).