Parallax makes an interesting set of chips they call Propeller chips. The original one is still sold, and likely to continue to be available for a long time, has a definite retro feel, despite being a 32 bit device and a concurrent multi-processor. Yes, one can do either parallel or concurrent type programming on it, and it shines when that is done.
Here's the interesting bit:
Each core has it's own private memory space. On the original design, assembly language programs would only run in that space, limited to 512 long instructions. If you want, you can imagine it as a CPU with 512 registers that can contain instructions or data and you won't be far off the mark.
There is a shared memory space of 32Kbytes, and each of the cores has access to it in a round robin fashion.
Programming this thing feels a lot like retro computing does. People have made operating systems, emulations and all the usual things, but what I like most is just building up projects, writing applications and experimenting with it much like I did (and still do on occasion) with my Apple 2. The difference is all those cores turn into soft peripherials easily.
Here's an example:
Say I need a TV text display. Take one of those cores, they call them COGS, and knock out a composite video signal, 320x200 and supply an 8x8 font, and there you go. 40x24 text, with the video buffer being whatever format, where ever you want it. One of my first projects was to do exactly this and I mooched the Atari font out of my Atari 400 ROM, got the signals just right, and it's very hard to even tell the difference at a glance.
Once something like this is done, integrating it with some project is super easy. In fact, it's probably the easiest I've ever seen it.
To the rest of your project, that core running appears just like a custom chip in an old computer does. You can have it read memory, act on values, signal VBLANK, and all the good stuff. From there, you setup the other bits you might need, sound, serial, and then program on a fast, currently available system (dev boards are quite capable), doing many things just like you would back in the day, and there is only as much of an OS as you want there to be.
The current chip is considerably more capable, offering higher speeds and crazy luxury in terms of I/O and hardware assists to help with things like capturing signals, or creating them.
If your retro interest extends into programming and electronics, or you want to combine stuff available today with or into your retro projects, these devices are an awful lot of fun.
In my view, the Propeller 1 is definitely retro feel. You can get one in a DIP 40 package, slot it into a breadboard Ben Eater style, and have something dancing on the screen, or blinking lights, making sounds in very little time and not too much effort.
The newer one is a capable system on a chip, and a few users have mentioned "The Amiga of Microcontrollers", which is a description I feel isn't too far off.
Some assembly required though. Seriously, both chips shine in assembly and offer a beautiful, simple assembly language, despite being multi-processors. And you will either want to make your own little system, or get one of the more capable dev boards to get going. If these things are interesting, or don't scare you off, it's all a good experience.
Here are a few demos made on the original design. I expect to see, and will likely contribute to similar productions for the new design, but it's still really new:
This one is done in Apple 2 type artifact color. Video authored by Eric Ball, animation by me, sound by the SidCOG author.
This one is in VGA, authored by a well known scener.
An accurate SID emulation, capable of playing pretty much any SID tune out there. There is an Amiga mod player under construction for the new design, which will be based on a serious sound engine near completion.
Not an emulation. This is a port of the 8bit game using a TV signal driver that outputs an Atari 8bit like video signal.