RIOT-OS
I've just stumbled on RIOT-OS , because it claims to run on an STM32F030, and I have one of those minimum system boards you can get for £1 on eBay, and I was wondering what open source support there was for it. It has a design principle I'm keen on: it's batteries included. I has a micro kernel, but also provides consistent APIs for a number of platforms. The list is impressive. Anyway - time will tell: MBED tries to do this too, but it's resource requirements are too great for many of the platforms listed by RIOT-OS, which suggests it's a lot leaner than MBED.