The SparkFun Arduino ProtoShield PCB and ProtoShield kit lets you customize your own Arduino shield using whatever custom circuit you can come up with! This tutorial will go over its features, hardware assembly, and how to use shield.

If you're ready to step up your Arduino game from older 8-bit/16MHz microcontrollers, the RedBoard Turbo is an awesome alternative.

SERCOM (Serial Communication) is a multiplexed serial configuration used on the SAMD21, SAMD51 and other boards. It allows you to select various serial functions for most of your pins.