Very early on in our estimation of this house we both agreed that the original fireplace mantel shelf needed to be replaced. Every single house around here had the exact same mantle, just a large chunk of wood nailed to the wall with 4 nails. Nothing fancy, as you can see by this picture:
Originally we were going to pay somebody to either build one or purchase one. The biggest problem with the purchase route turned out to be the size of the mantel shelf, it had to be about 8' long, and didn't have end caps just a 45 degree angle. None of the local hardware stores carried any premade kits larger than 6'. We were able to find something online we liked at mantelsdirect.com (the concorde model), but I wanted to give the business to a local store. You know, support the community and crap like that. So we drove around and looked, the prices were pretty high generally, and the designs were nothing we liked. My personal favorite was when I walked into a local store, saw a picture of the concorde model we liked and asked if they could replicate that for less than we could order an 8' version online (roughly 320). I had to talk them down about the price, but that didn't bother me. What bothered me was the draft they submitted. It looked like a retarded 5 year old had drawn it... Despite my desire to not spend any time on this it became clear we where going to have to make our own to get what we wanted.
So after some drawing and figuring out what we wanted and a little chance taking we were finally able to find all the parts we needed locally and threw together a mockup:

This is their proposal:

This is our early sketches:
All the wood is poplar simply because it's the cheapest quality (knot free) you can get and since we're painting it there's no reason for oak/pine/etc. Jen and I both signed off on the design after seeing the mock-up in place on the mantel and we bought the wood necessary and extra router bit (didn't have a 1/4" beading bit). I already had some 2x4 and some quality 1x6 lying around so we bought:
1 1/4 beading bit $14.98
1 1x2 poplar trim $4.72
1 2x2 $1.88
1 1x8 poplar board $18.32
1 4 1/4" x 8' crown trim $14.00 So if you want to total things out, our mantel cost about $53....woulda been $40 if we hadn't had to buy a router bit. Kind of a far cry from the $320 the local place wanted for their crap (didn't even have crown molding in it).
So now that we have all the wood all we have to do is assemble it :)

Now that all the parts that could be preassembled and painted are done we move on the assembly (which is actually easy as hell).
We cust the ends of the 2x4 backer to 45 degrees (and the proper length of course) and put it into 4 wall studs using 20 penny nails (two each stud, top and bottom). Let's just say that it's not going anywhere anytime soon. Now that the primary backer is secured, we can bust out the nail gun and have some fun. I used 2" 16 gauge nails for everything but the crown molding (1 1/4 18 gauge) and plenty of liquid nails to make sure nothing will ever move.
After the 2x4 backer was on we attached the 1x6 we cut down to match the backer's height with the additional 1x2 on it (5"). Once that was on we placed the mantel top on to see how much we'd have to "float" the 2x2's to make the shelf top level. Then we started at one end and used nails and glue to make sure they weren't gonna move horiziontally, and then secured it vertically from the top down into the 2x2's.

Now that all the 2x2 backers are secured we were able to cut the crown molding, which turns out to be very simple because it uses the same angles on the compound miter as the base caps for the pilasters. Once it's cut we switch nailguns to a 1 1/4" 18 guage brad...add a little liquid nails, and go to town. Once that's complete we're done....painting and filling of the holes will come later.
New mantel for about $50 bucks....not to shabby. Of course special thanks again to our neighbor Tim for lending us his router this past month. One last final comparison: