What are beer kegs made of?

Answer 1

Aluminum. Although there are still a few breweries (Miller) who also use rubber-coated kegs, although the inner drum is still aluminum. There are also wooden casks, although commercially-sold beer is not sold in these vessels - it is transfered into aluminum cooperage first.

Actually most kegs are stainless steel. There are a few aluminum ones out there, but due to the abuse they get the vast majority are stainless (even the rubber-coated ones)