I have literally been stepping on it for a few years now. There is a very large rectangle shaped piece of solid steel (3.5' x 1' x 1') that has been a step into my garage that I have literally stepped on almost everyday for the past 4 years. I have been looking for the past 2 years or so for an anvil and it never occurred to me that it could fit the bill. The problem is that it has very rough surfaces all the way around due to sitting out in the weather for several years. Is that a problem at all?

How rough? Deep pits? How are the edges?

That's a mighty big chunk. The end of it would be enough for blade forging if it was in better condition than the rest.

The pits aren't too deep, probably 1/16th'' deep. The edges are about the same.

If it helps at all, it has a ring to it when struck with a hammer and has some rebound to it also.

You can dress the edge and surface with a flapper disc and may have yourself an anvil.  

I'll look a home depot tomorrow for a flapper wheel for my angle grinder. Thanks for the advise!

Outstanding       I recently did the same thing.  I found a block about 3 1/2" x 7" x 18" in a scap pile at work.  I haven't done any forging on it yet but I'm sure it'll work fine for most knife making.  It has 5 bolt holes running crossways when laid on edge and I'm thinking I may be able to forge a small horn that I could bolt to the side latter if I decide I need one before I can afford a real anvil.

I've got a 6 lb. sledge head, a 12" RR Rail and a piece of 4140 cylindrical shape 6.25" diameter by 5.5" high that I'm trying to figure out how to make all three come together as something to beat on. Good luck with yours, let us know how it works out for you.