The Digispark is a very simplistic development board based on the ATTiny85. It's popular due to the Arduino compatibility, built-in USB-A plug (so no dangling cables!), the affordable price, and the fact that it can act as a keyboard.
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TinyDuck is another small dev board. It's specifically made for being used as a BadUSB, and since it's based on the same chip, it's fully compatible with the Digispark and this converter.
You can learn more about the project on GitHub.