Brownells have developed an online AR-15 builder flash application. It is not the first but the ability to add the items to your Brownells shopping cart and order everything in one go is a killer feature.
It says it is in beta and it really is. The trash can does not work and so it appears removing items is impossible, you can only replace them. Some items can be moved around, while others cannot. Adding an item from one category can delete an item from another category. I have emailed Brownells. Hopefully they will fixed them.
Overall it is a great start and I think Brownells will easily recoup their investment.
Use the application here.