I often see some original design on sale from Aliexpress, which are remix of other open hardware designs or brand new things. When I can I will make all efforts to try to find the original design, to support the inventor.
Price, indication and images may change without notice.
ATmega32u4 usb development board ~4,80€
This board has inside same processor as Leonardo and Micro, which means it doesn’t need at all a USB-serial converter, can do input as mouse or keyboard, and can be plugged directly on a usb port.
As a simple project you could make a password manager. Or a monitor auto brightness adjustment.
It runs at 8MHz instead of 16MHz, but I’m sure that for most projects it doesn’t matter as it can listen to buttons, do math, give commands and have plenty of cycles remaining.
These little boards are the closest you can get to a plug-play-deploy arduino you can get. While they can be programmed with Arduino IDE, they need an addon, don’t support Serial, and have >8Kb flash for programs.
Anyway, if you just need a simple driver chip for a beeper, flash some lights, testing a servo, or a PIR sensor these are for you.
Can be combined with a battery that has a usb output to make a project on the fly, like a PIR sensor with a buzzer to warn if motion happens.
The original design comes from Digispark, and was remixed and improved by Adafruit.
NodeMCU V2 ~6,40€
Even if it is not the cheapest NodeMCU boards, these have a smaller width, so they can fit on a breadboard, and have a spare row to plug in male jumpers.
Compared to ATmega based Arduino boards it is a whole new world:
-1Mb Flash memory for program and data-80MHz speed
However to get these bonus you lose 5v pin compatibility, only 1 analog pin which doesn’t stand more than 3.3v, less pins, no 5v regulator.