ZX-calculus
The ZX-calculus is a graphical language for reasoning about quantum algorithms. See this website for more information. The Wikipedia article actually gives a pretty good overview.
PyZX is a Python library which implements the ZX-calculus for quantum circuit visualisation and optimisation. Read the docs here.
ZXLive is a graphical tool for working with the ZX-calculus.
The Qiskit ZX transpiler is a transpiler pass for Qiskit which uses ZX-Calculus for circuit optimization, implemented using PyZX.
I’ve been contributing to these projects as a part of my effort to learn the ZX-calculus. I’ve also made a shared Anki deck with my study notes.