While the Bible does talk about slavery, that doesn't necessarily mean it supports it. As citizens, we obey the laws even though we don't necessarily agree with them. The Bible teaches that we should obey and respect the laws of the land, however imperfect they may be. It does not teach that slavery is right but merely that we should respect our leaders. The Bible also has many verses that would seem to led away from concepts like slavery as well. We have to consider the history and the time in which the Bible was written just as with any other literary text. We also have to take the work as a whole and not remove pieces out of context in order to understand the big picture.
The point is that there were many passages in the Bible that could be seized upon to justify slavery, as Post 2 points out. The Bible is a seminal work in western society, and since some Christians have often argued for a literal interpretation of the Bible, and viewed it as a moral guidebook without much allowance for its historical context, it was not difficult for them to use it to justify slavery. Southern defenses of slavery were full of references to precisely those passages listed above, as well as to bizarre theories about black people bearing the curse of Ham. Oddly, these coexisted quite nicely with contemporary racial theories that were not biblical but scientific in origin.
The Bible was written during a time when slavery was the norm. There are plenty of things that are in The Bible that we no longer accept. The Bible describes slavery because there was slavery. That does not mean that slavery is a good thing, or supported by God.
It supported slavery. There are numerous places in the New Testament (not just the old) where slaves are told to obey their masters. Colossians 3:22, Ephesians 6:5, 1 Peter 2:18.
There is never any discussion of slavery being wrong when done to others. God wanted the Israelites led out of slavery in Egypt, but that was not because slavery was wrong. It was because the Israelites were his people.
I think posts 2 and 4 are subtly referring to the book in the Bible called 'Philemon', which is a letter to Philemon, a slave owner, written by the apostle Paul. The situation was that one of Philemon's slaves, Onenesimus had run away from Philemon to Paul. Paul was doing as he should do, sending Onesimus back, telling Onesimus to be submissive.
However... Paul pleads with Philemon to set Onesimus free. Remember the sin I set you free from, Paul says. Put it on my account, because you owe me so much. It also reminds me of the parable of the Unforgiving Servant, when one servant owing much is forgiving all debts, but then turns around and demands a very slight debt from another slave.
I think because God has set us free from the slavery of sin, so we should not ensnare others in slavery, but also give them freedom.