To answer wishes of numerous Unity developers, a polygon reduction plugin for Unity Pro called Cruncher is now available. Created by RealWorldUnity, Cruncher is an efficient, robust and easy to use inside the Unity editor. Being a native Unity plugin that utilizes Atangeo Balancer SDK (http://atangeo.com/products/sdk) for mesh simplification, Cruncher can't be used right now with the free version of Unity.
See more details here: http://atangeo.com/products/cruncher
It is available directly from the Unity assets store here (http://u3d.as/content/real-world-unity-com/cruncher/3j7).
The supporting thread on the Unity forums is here (http://forum.unity3d.com/threads/149948-Cruncher-Polygon-Reduction-Plugin-for-Unity-Pro).