NSIS (Nullsoft Scriptable Install System) is a professional open source system to create Windows installers. It is designed to be as small and flexible as possible and is therefore very suitable for internet distribution. Being a user's first experience with your product, a stable and reliable installer is an important component of successful software. With NSIS you can create such installers that are capable of doing everything that is needed to setup your software.

NSIS is script-based and allows you to create the logic to handle even the most complex installation tasks. Many plug-ins and scripts are already available: you can create web installers, communicate with Windows and other software components, install or update shared components and more.

Bigger list of features
Generates self-contained executable installers
Support for ZLib, BZip2 and LZMA data compression (files can be compressed individually or together)
Uninstall support (installer can generate an uninstaller)
Customizable user interface (dialogs, fonts, backgrounds, icons, text, checkmarks, images etc.)
Classic and Modern wizard interface
Fully multilingual, support for multiple languages (including RTL languages) in one installer. More than 40 translations are already available, but you can also create your own.
Page system: You can add standard wizard pages or custom pages
User selection of installation components, tree for component selection
Multiple install configurations (usually Minimal, Typical, Full), and custom configuration
Installer self-verification using a CRC32 checksum
Small overhead over compressed data size (34 KB with default options)
Ability to display a license agreement in text or RTF format
Ability to detect destination directory from the registry
Easy to use plug-in system (lots of plug-ins for creation of custom dialogs, internet connections, HTTP downloading, file patching, Win32 API calls, etc. are included)
Installers can be as large as 2GB
Optional silent mode for automated installations
A preprocessor with support for defined symbols, macros, conditional compilation, standard predefines
A lovely coding experience with elements of PHP and assembly (includes user variables, a stack, real flow control, etc.)
Installers have their own VMs that let you write code that can support
File extraction (with configurable overwrite parameters)
File/directory copying, renaming, deletion, searching
Plug-in DLL calling
DLL/ActiveX control registration/unregistration
Executable execution (shell execute and wait options)
Shortcut creation
Registry key reading/setting/enumerating/deleting
INI file reading/writing
Generic text file reading/writing
Powerful string and integer manipulation
Window finding based on class name or title
User interface manipulation (font/text setting)
Window message sending
User interaction with message boxes or custom pages
Branching, comparisons, etc.
Error checking
Reboot support, including delete or rename on reboot
Installer behavior commands (such as show/hide/wait/etc.)
User functions in script
Callback functions for user actions
Completely free for any use.
And more...