Ganglife is alive in Harlem. I think that the Latin Kings were the first gang that I actually heard someone besides a rapper talk about. Other than that, I think I learned most of what I knew about gangs from listening to Cypress Hill.
It's surprising the resources that are out there about rappers and their gang affiliation. A lot of it overlaps and a lot of it is conflicting, but if you just look here, here and here you'll learn a lot!