Well I focus mainly on commercial however at this time I am not accepting agents.Some of the big dogs are CB richard ellis,ellis and grubbs,cushman and wakefield,marcus and millchap,ag armstrong,nai brennan goddard,the shopping center group etc.
There are many avenues to pursue in commercial and many big players as well as small firms.I couldn't list all that you could do in commercial it would take books to explain all fo the different niches.It takes very specialized training and you have to decide what you specialize in.
Inustrial,retail,land,office,leasing,development,apartments,hotel,restaurants,business brokerage to name just a tiny few.
good luck