While searching for a clean copy of the US Constitution on an official site, I found a set of six amendments that have not been ratified by the states. A summary is here, including the pro-slavery amendment and the DC statehood amendment.

The history is that two were proposed with the Bill of Rights and were not approved (although the second one was proclaimed approved in 1992), then in 1961, 1924, 1982, and 1985.

• An amendment regulating the number of representatives and how they are apportioned.
• A prohibition on a US citizen holding an appointment, office, or title from another country. If this had passed, anyone receiving an honorary knighthood would have immediately lost their citizenship.
• In 1861, two days before President Lincoln's inauguration, then-President Buchanan signed (unnecessarily) an amendment later not ratified to prohibit the creation of an Amendment abolishing slavery.
• 1924. An amendment giving Congress the power regulate child labor.
• The ERA in 1975.
• An amendment proposing DC statehood (for the purposes of "representation in the Congress, election of the President and Vice President, and Article V of the constitution," where Article V is the creation of amendments to the constitution. This would have also repealed the 23 amendment, which gave District residents the right to vote in Presidential and Vice-Presidential elections with the same number of electoral votes as the smallest state, or fewer.
• The Congressional Pay Limitation amendment, proposed with the Bill of Rights, and declared to still be waiting for ratification until 1992. Originally proposed in 1789, it was ratified by six states between then and 1791. Then there was a drought until 1873 when Ohio ratified it. And then another drought until 1978 when Wyoming got the ball rolling again, resulting in ratification in 1992.