Foreign Policy lists them, I’d only heard of one, Mara Salvatrucha. Their organizational styles appear to be heavily decentralized, using 4GW tactics, which makes them difficult to monitor and infiltrate. John Robb once blogged (don’t have link) that Mara is transnational and uses extremely sophisticated money laundering operations.
Cash businesses are the best for laundering money. I suppose if a gang had serious resources, they could always buy a bank, and simplify things even more.
In The Godfather movies, the Corleone family eventually wanted to go legit, or at least have most their businesses that way. Will the major gangs of today go that route too?