I contribute to a number of open source projects – software that is free to use and free to modify.
DFUProg
Short for Device Firmware Update Programmer, this is an application that allows you to update the firmware in microcontrollers using the DFU protocol over USB. Most USB microcontrollers from Atmel are supported, and some from ST.
You can find this project on GitHub.
ES-Printf
This is a version of the standard C printf function adapted for use in small microcontrollers with very limited memory. It is highly customisable, allowing only the required features to be included to save memory and execution time, and with a custom output function. Originally written with Atmel AVR microcontrollers and the WinAVR/GNU compiler in mind, it can easily be adapted to other devices and compilers.
You can find more information here and the project files are hosted on Sourceforge.