This place was one of my first experiences to building your own gun.
http://www.roderuscustom.tzo.com/
He also does a http://www.diyguns.com/

What is an 80% 1911?

Well, I didn't understand it either. If I understand it corerectly it is a 1911 minus a serial number. Which means it doesn't have to be registered. I'm not an expert and I have nothing that verifies this. If anyone is well versed in this I'm all ears.

1528.8

Yeah, basically this. The frame is basically /almost/ there. You cant make it and sell it, but you can use it for yourself. It would be a way to build a completely "clean" gun (As in lines not history.) No stamps, just pure finish able metal.

I've been wanting to do an AR Lower myself, just so I can doodle on it with the CNC machine and make it all fancy.

Well played sir!

An 80% receiver is completed to 80%. There are no serial numbers due to the fact that it is not a completed firearm. You must build it yourself and complete the last 20%. The last 20% on a 1911 frame is cutting the rails and drilling some holes. The receiver can not be legally sold to anyone and must always remain in your possession. There are several 80% receivers for lots of different guns and they are for the home builders out there such as myself. The down side is that you do not really save any money by buying one and you still have to do quite a bit of work. With the cost on stripped receivers as low as they are for most guns it really isn't worth it.