Check out www.AR15Builder.com
It just went live this morning. There are well over 600 individual choices of materials and accessories in this system. You can either drag and drop to create the AR of your dreams or you can just click on the items and they will be added to the display, showing you what the actual rifle will look like. Real photos were used to create the program so the representation on the screen is as real as it can get.
Brownells has hit on a really koooool way to shop for the accessories or the major components you want in your AR, on top of that it automatically creates a shopping cart list so you can see what it will cost and go direct to placing an order. This is bar-nun one of the best web site tools I have seen in a long time.