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Armstrap Claims to Make ARM Prototyping Easier (Video)
It almost seems too perfect that the originator of the Armstrap 'community of engineers and makers' is named Charles Armstrap. He just introduces himself as 'Charles' on the Armstrap.org website. Names aside, Armstrap.org is 100% open source, including circuit board designs. This is not a 'draw your own circuit boards' bunch, although you certainly could if you wanted to badly enough since they provide schematics and even full CAD drawings of what they make. The reason they do this is laid out on their Core Values page. The boards Armstrap sells are not expensive, but if you are going to be truly open source, you need to supply the means to duplicate and modify or extend your work, as is totally permitted under the MIT License they use.
ST makes several ARM M4F based dev/eval boards with built-in JTAG and a few additional chips thrown in to play with (I think accelerometers and MEMS microphones are common). They cost around $15-$20... go to http://www.st.com/web/catalog/... and check the box for STM32F4 under Supported Devices.
So, with what I suspect is the benefit of manufacturer loss-leader subsidies on the Discovery boards, why would I spend $40-$60 more on a dev board?
ARM has two general series of processors, the A series, or application processors, and the M series, or microcontrollers. Microcontrollers (such as the STM32F4 in the Armstrap boards) are designed for low-power embedded device use, you *might* run a specialized RTOS on one, but usually it's just your code, running bare-metal. An application processor, such as what you have in your smartphone or in the Pi, is a more general purpose computing core, running faster, taking more power, and using a full OS for resource management and process scheduling. Also, generally, microcontrollers have their own built-in RAM and Flash, while application processors use external RAM and storage (the Pi looks like the RAM and CPU are one, but in reality they're two separate dies, stacked up within the same package, while micros have everything on the same die).