I've heard that Rabeinu Gershom (a 10th-11th century scholar) banned 3 things, and I know one of them is polygamy. Does anyone know what the other two are?
Be'er Hagolah at the end of Yoreh Deah 334 lists the following:
- Not to marry more than 1 wife.
- Someone who is in Cherem should not be a Shaliach Tzibur.
- A person should not be away from his wife more than 18 months.
- Not to rent a house from a non Jew if a Jew is living there.
- Not to cut off the margin of a page of a Sefer, even to write on it.
- Not to embarrass a Baal Teshuva about his previous sins.
- Not to look at his friends letters without permission.
There are many more, see the link above.