Ocean - Canada, dubs everything but only gets a few anime series (Death Note, Black Lagoon, InuYasha, Gundam)
BangZoom - California, dubs most popular series. Recycles the same voice actors for each series.
Studiopolis - Basically BangZoom, except uses more actors.
Funimation - Uses in-house talent, Texas. Lots of actors. I think they only do anime.
Hopefully Viz send Monster to Ocean whenever they decide to fucking ANNOUNCE their license, since they took it to that one conference like a year ago and haven't said anything about it since.