Oh gosh, slavery's everywhere, even today. And even before the infamous African American slavery that happened way back then in the old USA. Doesn't matter on skin color or race.
It's hard to pinpoint which century slavery has happened, because I believe it happens in nearly every century. It may have been viewed as hard voluntary labor back then, or forced labor (either temporary or until the slave dies).
In a way, we're all slaves to some higher power, if you think about it. xD; Being a slave just turned into being an average everyday worker.
But if you're talking about extreme circumstances, I do believe even African people were slaves amongst themselves. Not to mention the feudal/ancient eras in some regions of Asia, they also used slaves. Oh, and let's not forget about the Germans infamosy of using the "impure" people (such as Jews, gypsies, anyone that didn't fit their "perfection") for those cruel labor camps. It's not only Germans though; I'm sure nearly every country in Europe dabbled in the usage of slavery at some point in time. South America had some slaves too, although I have very limited knowledge on that. The only continent I'm not sure about is Austrailia. xD