Poor tithe

The poor tithe, or poor man's tithe (Hebrew: "ma'sar ani"), also referred to as the pauper's tithe or the third tithe, is a triennial tithe of one's produce, required in Jewish law. It requires that one tenth of produce grown in the third and sixth years of the seven-year sabbatical cycle be given to the Levites and the poor.
The law applies during the days of the Temple in Jerusalem, and after the Temple's destruction. It applies only to crops that are harvested in the Land of Israel, but during the Seventh Year, also applies to crops harvested in Jordan and Egypt, so that the poor of Israel would be supported in the Seventh Year.
In the Hebrew Bible.
The poor tithe is discussed in the Book of Deuteronomy:
Thus, this tithe is separated from homegrown crops during the 3rd and 6th year of the seven-year cycle. 
The early rabbis, the Tannaim and Amoraim, understood these texts as describing two separate tithes: the first tithe (Hebrew: "ma'aser rishon") to be given to the Levites and the second tithe (Hebrew: "ma'aser sheni") in Leviticus to be kept by its owner and to be eaten in Jerusalem, except in the third and sixth years of the sabbatical cycle, when instead of separating the second tithe, the poor tithe ("ma'sar ani") was separated and given to the poor. The medieval commentator Rashi also interprets Deuteronomy 26:12 as referring to the third year, when the first tithe was given to the Levite and the poor tithe was given to "the stranger, the orphan, and the widow".
In Josephus.
Jewish-Roman historian Flavius Josephus refers to the first, second, and third (or poor) tithe. The third tithe was to be brought to the Levites, every third and sixth year of the seven year Sabbath cycle. The distribution of which to be given to those in need or want, especially widowed women and orphan children.
In the Talmud.
The Babylonian Talmud ruled that the amount of poor tithe one gives to a single poor person must be enough to provide for two meals.
The Babylonian Talmud also states that while "ma'sar ani" technically could be used to feed one's poor father, one should not do so, so as not to embarrass one's father. Rather, a son should try his best to care for his father out of his other property.
The Jerusalem Talmud to Pe'ah 1:1 discusses the maximum amount of one's income/money one can give to the poor and determines that one should not give more than one-fifth of his possessions so he does not become poor himself. This Gemara and a discussion in Sifrei are quoted extensively by later Jewish sages who discussed an ancient custom of tithing 10% of one's income for charity. This tithe, known as "ma'sar kesafim", has become a universal practice.
In Orthodox Judaism.
There has been no organized mechanism for collection of the poor tithe since 135 CE, but Orthodox Judaism still regards tithe obligations as residing in produce grown in the Land of Israel. Contemporary practice is to set aside "terumah", separate first tithe ("ma'aser rishon"), separate "terumat ma'aser", separate either the second tithe ("ma'aser sheni") or the poor tithe (depending on the year), then (if applicable) redeem the second tithe with a coin.
Orthodox Judaism regards it as meritorious to discharge one's poor tithe obligation additionally by giving a portion of one's income, ideally a tenth, to charity.