What are the most popular online commodity brokerage firms?

Jimmy J asked:


Having invested in stocks for quite sometime, I am thinking to get into the commodity trading world. As a beginner in futures and options on futures, I would like to seek advice on the names of brokerage firms for such investment. I wonder what are the most popular online commodity brokerage firms (like TDAmeritrade for stocks) for setting up “self-directed” accounts with reasonable commissions, fees and requirements for opening accounts. Many thanks.