It's highly debated, but the Corleone family is (I think) supposed to be the 5th. They're just referred to as the "Five Families" even by their own members, so people speculate there's another one.

Incidently, though, the novel mentions a sixth family in the chapters that talk about Vito's rise to power ("There were five or six families too powerful to eliminate"). This family isn't named and all it really says is that Luca Brasi assassinated the head of the family, and Vito made peace with them later on.