NSIS (Nullsoft Scriptable Install System) is a tool that allows developers to create installers for their software. It is open-source and highly customizable, with support for various installation types and languages. NSIS is known for its small size, fast performance, and flexibility, making it a popular choice for creating installation packages for Windows applications. Additionally, NSIS offers a plugin system that enables developers to extend its functionality and integrate it with other tools and services.