The Pro Micro has become known as the "standard" when building DIY keyboards. Though other, newer boards have been developed (such as the Elite-C and Proton C; both sold on SpaceCat Design btw!), the Pro Micro continues to grow in popularity due to it's continued QMK support and functionality and an amazing community of people developing for it. Uses a standard ATMega 32U4, color of board PCB may change from time to time.
Original concept for board by SparkFun.