SDKs

Python

matrix-nio (Python)
mautrix-python (Python)
µtrix (Python)
Simple-Matrix-Bot-Lib (Python)

JavaScript

Matrix.org JS SDK (JavaScript)
botkit-matrix (JavaScript)
matrix-appservice-bridge (TypeScript)
matrix-bot-sdk (TypeScript)
smallbot-matrix (TypeScript)

Go

gomatrix (Go)
mautrix-go (Go)

Rust

matrix-rust-sdk (Rust)
Ruma (Rust)

Ruby

Ruby Matrix SDK (Ruby)

Java

Matrix.org Android SDK (Java)
Matrix-ClientServer-API-java (Java)
jmsdk (Java)

Objective-C

Matrix.org iOS SDK (Objective-C)

Dart

Perl

C/C++

libkazv (C++)
Quotient (C++/Qt)

Kotlin

Trixnity (Kotlin)

.NET

MatrixAPI (C#)
Matrix C# SDK (C#)

Other

Matrix::Client (Raku)