The nxp LPC1114 is an inexpensive, capable ARM processor.
One variant (the LPC1114FN28/102) comes in a breadboard-friendly 28-pin DIP.
It has 32k bytes of EPROM and 4k bytes of RAM. Its hardware support includes serial communications, I2C and SPI.
It has 6 ADC channels and 22 GPIOs.
It needs no external components, it's easy to program over a serial connection, and its digital inputs are 5v tolerant.
The production of the DIP version of the LPC1114 has stopped, and when current stocks run out the chip will no longer be available.
Mouser still has a few and I found a few other vendors offering it - at a price!
Happily, there are good alternatives; see the Blue Pill section below.
I found several descriptions of how to wire it up and program it; they differed slightly, and one or two seem to contain minor errors. I'll try to describe what I did as accurately as I can and explain the choices I made during implementation
It's easily programmed using a USB-to-Serial cable, and there is a pre-built FORTH image available.
I'm currently exploring the combination and will document the hardware construction and software installation in this GitHub project.
The project was inspired by two great Hackaday articles, and an excellent post from Jen-Claud Wippler on his jeelabs site:
Here's a picture of my first breadboarded version:
Here's version 2.0, built on one of Adafruit's wonderful Perma-proto boards.
You can read about a widely available and inexpensive DIP-friendly ready-made board with a more powerful ARM chip on the jeelabs blog.
Like the LPC1114 the chip can be programmed over a serial connection. The board has an FTDI on-board USB-to-serial converter, so all you need is a USB cable and the appropriate software.
Apparently the board is often referred to as the Blue Pill.
It's easy to find at a cost of $2- $3 dollars. Just search for 'stm32f103c8t6 board' on eBay.
As with the LPC1114 there's a mecrisp FORTH image available. I have some boards on order and will report once I've had a chance to investigate.