well lets see, there's quite a few options for people like me when i get in the mood to do something like that (by people like me i mean those with no formal training and nothing but a few, very very few tools)
let me first start by saying with a little bit o' creativity your options are limitless you can build anything you can think of.
some projects are a little easier than than others. let me try and list these main options starting with ease of construction. but first let me differentiate the "firearm" from the "parts kit." there is only one piece on a working gun that is considered the firearm is really the frame or receiver, which is controlled in sales and individually serial numbered. the rest are just spare parts, no different than anything else you buy like tools or groceries
first and easiest, buy all the parts of a gun and put it together, for this you need an ffl to transfer the frame to you and get the rest of the parts from wherever probably somewhere from the depths of the internet. the drawbacks to this are that you must take the background check, and you now have a regular gun, same as if you bought it whole, b/c of the frame it is linked to you by serial number (i don't want to say registered b/c supposedly there's no "registration" just a method in place to trace guns back to their owners) the benefit here is that you normally wont need any machines and just basic hand tools to have a working firearm.
next you can get a frame (or receiver) that has only been machined 80% of the way, since it is only machined 80% the batfe doesn't consider this a firearm, meaning you can order them sans 4473 and have an off paper firearm. now it starts getting tough, dependent on what you want to build. you have to be able to complete the last phases of machine work.
a. for the ak frame you have two basic options, you can buy a flat piece of metal that was machined exactly like and ak receiver, but flat, you have to put two 90 degree angles into it and weld (or rivet) the bolt rails.
b. you can buy a bent piece of metal but that has none of the required slots and holes put on it.
with 80% or less receivers you have lots of options, you can build an AR, AK, CETME, FAL, sig 22x, beretta 92, 1911, 10/22 and probably a bunch of others oh yea, Thompson sub machine gun, stens and other tube guns as well.
next you can get really creative and just wild out making everything at home, including the frame this is the hardest and i ran out of time so you can look it up more yourself
go here for excellent info, you have to register to view all the forums
www.homegunsmith.com
i especially recommend the "hall of fame" forum and the "ask a guru" forum. but you have a lot more reading to do
www.thehomegunsmith.com
http://ktordnance.com/kto/
http://www.tanneryshop.com/
www.ar15plus.com