Zakat recipients are enumerated in the sixtieth verse of Surah al-Tawbah:
إِنَّمَا الصَّدَقَتُ لِلْفُقَرَآء وَالْمَسَكِينِ وَالْعَمِلِينَ عَلَيْهَا وَالْمُؤَلَّفَةِ قُلُوبُهُمْ وَفِى الرّقَابِ وَالْغَرِمِينَ وَفِى سَبِيلِ اللَّهِ وَابْنِ السَّبِيلِ فَرِيضَةً مّنَ اللَّهِ وَاللَّهُ عَلِيمٌ حَكِيمٌ
Charity is only paid for: the destitute, the poor, those collecting it, to soften the hearts; in manumission, those in debt, in God’s path, and the wayfarer; an obligation from God. God is Omniscient, All-Wise
Zakat, given the generality of this verse, is to be used in the freeing of slaves and generally the abolition of all forms of human subjugation. It is important to note here that peonage and human trafficking still exist today, and most societies are guilty of illegal bondage, even Muslim ones. For resources and examples of how Human Trafficking still exists today, see here, here, and here. Zakat can and should be used to fight this form of injustice.
Point of interest:
While unrelated to the topic of Zakat, the following is an interesting follow-up issue to that of peonage and slavery. Al-Bukhari narrates from Abu Hurairah that the Prophet said:
قال الله تعالى ثلاثة أنا خصمهم يوم القيامة رجل أعطى بي ثم غدر ورجل باع حرا فأكل ثمنه ورجل استأجر أجيرا فاستوفى منه ولم يعطه أجره
God himself has said: Three people, I am their plaintiff on the Day of Judgment; a man that was given in my name, yet was deceitful; a man that sold a free man and ate the price; and a man that hired someone,and whence he received full service, did not give him his pay.
Chattel slavery (the type that comes to mind for most western audiences) is forbidden in Islam, due to this hadith and many other texts. Any freeman that is robbed of that freedom is due reparations. Most legal works designate restitution for a person who, held against his will, was forced to work for the enrichment of another person. Scholars of the Maliki school have stated that if a person were to kidnap and sell a free man into slavery and it was impossible to locate that person or know of his whereabouts (ie to return him to his family), then the guilty party should have to pay the blood-wit (diyah) in total to that person’s inheritors.
 – See Al-Hattab, Mawahib al-Jalil, and ‘Illish, Minah al-Jalil. The latter states this as the position of Imam Malik himself.
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