Small form factor low end STM32 dev boards. Like the bigger Nucleo they have a programmer/hardware debugger on board. Similar size to Arduino Nano etc, but with much higher performance, loads more peripherals, 32 bit processor, 48 Mhz clock etc
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/data_brief/DM00214578.pdf (http://www.st.com/st-web-ui/static/active/en/resource/technical/document/data_brief/DM00214578.pdf)
regards
Andy