I recently ordered a bunch of STM32F103 boards from China. They have a rectangular footprint like the Teensy or Arduino Micro, and a 48-pin STM32F103C8T6 MCU on board -- a 72MHz Cortex-M3 with 64kB of flash and 20kB of RAM. It's just the single MCU chip on board, with a bootloader, and the SWD pins brought out to a header, which should make it quite flexible; I'll be able to program/debug them with Eclipse and OpenOCD if I want full control, or I can use the STM32duino Arduino port if I just want to push code to them. The best part is that they only cost $2.25 each on Aliexpress, or about $3.50 on eBay, which is about half the cost of the chip alone when bought in the US.
The boards I've ordered are 2.1" long by 0.9" wide, with a rows of 20 pins along each side, and 0.6" from the center of one row to the other. I've made a KiCad part and footprint for them:
Part/library → myelin-kicad-libraries/stm32f103c8t6-module-china.lib
Footprint/module → myelin-kicad.pretty/stm32f103c8t6-module-china.kicad_module
These should be good for controlling LEDs over USB, or for soldering into projects where I feel that a Teensy would be too expensive.