Slavery is endemic to my homebrew campaign's setting, so much so I recently bought the _Chains and Misery_ pdf package from Skald Books to help decide various game matters, including prices and prestige classes. I haven't had a chance to really read it, yet, though, so I can't render an opinion concerning it.
My group's characters have at various times been slaves, owned slaves, and indulged in slaving. They have also fought slavers tooth and nail to rescue people from slavery. The practice is a neutral alignment factor for my group. All the nations in the campaign setting save one legalize slavery to some degree or another, using the old Roman and Greek models for comparison.