Maple Mini
There are some truly inexpensive looking boards floating around, based on the Leaflabs Maple Mini, but somewhat stripped down.
I ordered one and tried it out for a bit earlier. The board contains an stm32f103, USB port, power regulator, crystals, and pin breakouts.
There are several blog posts using these.
Toolchain on Debian (jessie)
I'm using all-integrated GCC and firmware tools. At the time of writing this, the versions are:
gcc-arm-none-eabi 4.8.3-9+
dfu-util 0.8-1
picocom 1.7-1
I've set up udev rules for permissions to the maple:
% cat /etc/udev/rules.d/49-maplemini.rules
## Maple Mini clone
ATTR{idVendor}=="1eaf", ATTR{idProduct}=="0003", MODE="0660", GROUP="plugdev"
ATTR{idVendor}=="1eaf", ATTR{idProduct}=="0004", MODE="0660", GROUP="plugdev"
Toolchain on Windows
I've also been toying with Eclipse on Windows. Stories are found in blog posts linked below.
ChibiOS
I decided to try running ChibiOS on Maple Mini and it does seem to work nice. That might be something to build on: $4 STM32 development board, USB ready and compact, full Free Software toolchain including USB firmware loader integrated in Debian, and a portable RTOS with a working board definition and demo.
There migh be pieces missing, of course. I've mostly only tested the build and self tests. Stories are found in blog posts linked below.