NGUI is an addon for Unity3D supporting developers to build enhanced UI systems like menus. It supports building powerful GUIs while rendering all elements within one draw call.
Citing from NGUI's web site http://www.tasharen.com (Oct 9, 2012):
NGUI is a powerful UI system and event notification framework for Unity (both Pro and Free) written in C# that closely follows the KISS principle. It features clean code and simple, minimalistic approach to everything. Most classes are kept under 200 lines of code. For a programmer this means a much easier time when it comes to working with the kit — from extending its functionality to tweaking the existing one. For everyone else this means better performance, less frustration, and more fun.