Hello everyone, it's been quite a while since my last project. So, since I had some "extra" time, the following is apparently what happens.

This is a simple re-purposing project for all that foam rubber out there. All you really need is an electric craving knife, some glue and a substrate.

Step 1: Step 1

So, first off, to make this worthwhile, you need to find a free or cheap source of foam rubber. Which is pretty easy, but of course you also want to make sure you're not inadvertently infecting your house with fleas, bedbugs or any other nastiness (Alex).
I found my inspiration for this project in the form of a futon mattress out front of an apartment that was being renovated. It looked fairly new and had a cover on it that was stain free. If you don't want to trash pick, you can usually get really cheap mismatched couch cushions from some retail furniture stores (this was actually how I furnished my first apartment.... two dozen mismatched couch cushions.... ah youth).
P.s. I know this pic is not a futon, I didn't take a picture of my find before I dismantled it, this is just an example.

Step 2:

Ok, now we need to take any covering off and get to the foam. Get a straight edge and a pretty long sharp knife and something to cut on. I used a ridged kitchen knife & a straight 2x4 on an old piece of carpet. Put some pressure on the straight edge to compress the foam a little bit and make several passes with the knife. You can also do this step with the electric carving knife, but you'll probably have to use a little different procedure.
I made my pieces so that the ends are square, this makes the next step easier.

I should have taken a picture of my living room when I had a hundred of these "foam fries" laying all over. Better yet, I should have taken a picture of my girlfriends face when she came home in the middle of this.

Step 3:

Now to process the foam strips into wedges, this has got to be one of the most wonky rigs I've ever assembled, but it did the job.

I taped some cardboard to a few right angle brackets to create a chute. Next I used some clamps to secure the knife to the table straight up and down. I then positioned the chute so the blade would cut right down the middle, added some support in the form of literature, groceries and a mini ironing board. You'll have to pull the foam though as it doesn't have enough rigidity to push it.

I credit this setup to the lumber mills in Skyrim, that f#%king game is incredible.

Step 4:

Almost there.
Take your wedges and arrange them on the substrate and glue! A friend of mine suggested that I try regular white elmers since it's cheap and there's next to no fumes, especially because I'm doing this in my living room and it's also the middle of winter.
I actually used corrugated plastic as my substrate so I had to sand it first to get the regular white glue to work. Rubber cement would probably be the optimum adhesive to use here.

Step 5:

After they are glued & dried, hang and enjoy!

There are lots of variations as well, deeper and or differently shaped wedges, different types of foam (although softer is usually better for acoustic damping).
You can also cover them in "acoustically transparent" fabric if you don't like the look of the wedges.

That's about it, have fun.
Love to my fellow Iblers (especially to those who got the Clock Work Orange reference in step 1).