https://raspberry.piaustralia.com.au/products/raspberry-pi-p...
Since I am a Go developer, I like using TinyGo with these type of boards, which means I get concurrency (goroutines) as well, although sadly Tinygo doesn't yet support the Wifi module. However, I've used other boards with Wifi, and once you're connected to WiFi you can poll APIs and do anything else you do on the internet. No OS required!
As an example of a complete, stand-alone program, I wrote a digital clock in TinyGo. (https://github.com/doctor-eval/clocky) - you plug the Pico into a 7-segment display and it operates just like an old school digital clock.
Sadly it can't (yet) connect to the internet due to missing wifi support in Tinygo, and it's running on a pre-W Pico, but you get the idea.
The cheapest you can get is about $5(Espressif assembled board), with full LED matrix controller.(that's the cost of manufacturing, though)
a full blown Linux stack is $10+
Genuinely curious, as I've typically used things like the Pi for projects like this but have been genuinely curious what it would take to strip it down to something less powerful and achieve the same result.