That is caled a half barrel, and is probably of the Sanke style. They are difficult to use for kegging, but MANY of us cut the top for our boil kettle. Simply cut about a 12" circle out of the top using whatever means you have access to (I used a plasma cutter, but many use an angle grinder, dremel tool, or even a sawzall), clean the edge up, and you have a 15.5 gal stainless pot.
That is a reall score to get one for free! look through some of the equipment photos in the gallery and you will see lots of these converted "Keggles."