Tales of the Inner Sea
Though all sorts of romantic pairings between races exist, the three which seem to most readily produce offspring are between Humans and Orcs, Elves and Halflings. Humans and halflings are similar enough that their common offspring are indistinguishable from short humans. Offspring of Elves and Humans are surprisingly common, and most likely to be found in Etro, Carcosa and the isles of the Inner Sea as those have the most diverse and cosmopolitan populations. Half orcs are a bit more rare, and are likely generally the product of the rape of those taken prisoner by the orcs. One would expect, if the elven origin story of Orcs is true, that they would more readily produce offspring with elves than humans. Unfortunately, due to the fact that elves are more valuable to Orcs as delicacies than slaves, it is difficult to say whether this is in fact truth.