UniKey is the most popular third-party software and input method editor (IME) for encoding Vietnamese for Windows.

It gained popularity for encoding Vietnamese thanks to its speed, simplicity, and reliability.

Since then, the engine has been integrated in input methods in different OSes and software frameworks.

ibus-unikey (developed by Le Quoc Tuan, using UniKey engine) is widely used for Linux distributions.

[1] From Mac OS X Leopard onwards, released in 2007, Apple has integrated the UniKey Vietnamese Input Method to the built-in Vietnamese input of Mac OS.