Sometimes different species of the same genus can come together to make a new plant, but usually that plant is unstable in terms of fertility. Ex: Horse + Donkey = Mule, but it is a sterile creature. Mint + Sage = Salvia (Salvia is simply the genus for "sage") A plant that was bred by Indians who (supposedly) had no knowledge of genetics. But it very seldom releases seeds, and survives mainly through cloning. My question is, what other plants can Cannabis breed with?
But you can also breed it with Hops successfully I have heard, and I'm sure that is not the only other thing.