I don't know much about Brooklyn Union Gas or Keyspan. If the company is still around you can sell it or hold onto it. If you just hold the stock certificate you can transfer it to a brokerage account and sell it (contact a broker to figure this out).
On the other hand if the company is not still around or your stock certificate is not valid for whatever reason you can see if it still has any collectable value. Here are some good places to do research on that.
