Programmable boards/microcontrollers for Wearables
There are a lot of options to choose from for the brains of your project. I've compiled a list of all the boards I could find that work well in wearable projects. You can find the list here on the main site, and I've reproduced it below, as well. Please let me know if I'm missing anything; I'd like this list to be as comprehensive as possible!
The slightly older version of the Circuit Playground Express. It works with Arduino and Code.org. At $20, it's $5 cheaper and has slightly reduced capabilities and hardware compared to the newer version.
The latest version of the Flora costs $15 and can be used with Arduino. It's larger than the Gemma and has more pins.
This is the latest version of the Gemma board; older versions seem to have been deprecated and may not even be available any more. It's $10, about the size of a quarter, and can be used with Arduino or CircuitPython. This is a great option for very small projects. It doesn't have many connectors, however, due to the small size.
This $10 board was inspired by the LilyPad line, but is made by TinyCircuits. It's smaller than a dime, yet contains a standard Atmega 328 processor, the same one that's used in the full sized LilyPad, Arduino Uno, etc. This board is also programmed via an FTDI connection; TinyCircuits recommends this adapter, which also costs $10. With as many pins as are available on such a small board, I wondered if it might be difficult to sew, but according to this review it is not. The review also recommends against using this in your very first project, due to the awkwardness of working with such a small sized board, but they do recommend it for more experienced makers.
Feather, ItsyBitsy, Teensy, Trinket
These are all very small boards available from Adafruit in a variety of different models that are sometimes referenced in wearable electronics projects due to their small size. They come at different price points with different features, but none of them are specifically designed with wearables in mind. This means they will be difficult (or impossible?) to sew, as the pads weren't made with sewing in mind. If you need something sewable, consider looking for a similar sewable board. E.g. the Gemma is very similar in functionality to the Trinket.