> Who out there is programming these chips in pure C using open source compilers and bootloaders?

The gcc-arm-none-eabi toolchain is pretty much what you are asking for at least for ARM targets. You can literally use a text editor and gcc-arm-none-eabi, that's it.

And if you want something really bare bones avr-gcc still targets the whole atmel family including those ATtiny chips which are also a lot of fun.

I don't know the state of it nowadays but 'Mbed' is probably worth looking into. The project had _a_lot_ of Middleware libraries to abstract hardware, a few levels below, makes embedded development a little less datasheet dependent, specially if you are just hacking something as a hobbyist.

You can also ditch the space consumed by a bootloader and save the UART for something productive in your designs. This is makes it feasible to use the smaller capacity chips and have more headroom on the larger ones. AVR programmers are cheap and the latest serial port based protocol requires the barest of hardware to support.