I just ordered one of these and it really worked out well.  BabySafetyFoam creates custom foam cover for virtually any fireplace hearth.  It's much better than the ones that only cover the sides & corners in my opinion because it covers the entire thing.  (Which is cool because it can double as a seat as well). 

I thought it was pretty neat that you can customize the cover to fit virtually any hearth shape.  You just measure the height, depth & width and submit them online.  In about a week you'll get your custom fitting safety foam.  It comes with plastic fasteners that really cling and it looks really clean and sleek.  (I'll admit I was surprised when it came unassembled, but I was able to put it together in about 5 minutes).

It's also fire resistant. It comes in one color - sort of a coffee with milk color. 

Prior to finding this, (an easy alternative) I went to a fabric store and purchased thick foam padding and essentially created the same thing and then I covered it with fabric by just wrapping it up.  That did the job for about 4 years.  The padding that I had on there from the fabric store was a lot thicker, but it didn't look quite as clean.

So overall, I really like this product and even now that the kids are a little old (6 &4) I feel safer about covering up the brick hearth to prevent injury.