No Cotton Gin

Point of Divergence: Eli Whitney's observance of an African slave cleaning cotton either 1) never happens or 2) never gives him the idea to invent a more efficient way to clean bales of cotton, and the cotton gin is not developed until the latter 19th century, well after the total abolition of slavery in the United States. In this alternate reality, much of history is different for African-Americans, the United States, the United Kingdom, Canada, Sierra Leone, and Trans-Atlantic relations.

Summary
With the lack of a cotton gin, the following would happen (as copied from my own answer on Quora - Harry):
 * With six slave states prior to 1793, all bound to the eastern seaboard but with large western expanses, the eventual territories and states created from their borders (Alabama, Mississippi, Tennessee and Kentucky) would not have carried slavery over into their self-governance by 1800. They would have abolished slavery prior to entering the Union.
 * Alongside the Midwest, the South would have been a more potent, almost year-round breadbasket during expansion into the West.
 * The other pocket of slavery in the South would've been Southern Louisiana, due to its own history of importing slaves from French and Spanish territories for sugar plantations. Americanization of slavery would not have happened in Louisiana, the Code Noir would be kept intact, resulting in a largely-local movement to abolish slavery.
 * With Mexico gaining Independence in 1821 and allowing U.S. settlers into Mexican Texas, those settlers would not have brought slaves nor established plantations in East Texas. The mainstay for anyone who wanted to make money in Texas would have been ranching out in West Texas while East Texas would have been a farming region.
 * Armed Maroon Africans who had escaped slavery in Georgia to reach freedom in Spanish-owned Florida and intermarried with Seminole aborigines would have been able to bide their time long enough to see abolition happen north of the border in the early 1800s, and Florida would have either stayed a swampy Spanish territory with slaves on the northern coastline and Maroon/Seminole settlements in the interior or would have become U.S. territory much later.
 * The Trail of Tears might not have happened.
 * The African-American population would not have quadrupled in size, and would've remained concentrated along the Eastern seaboard states.
 * Manumission and gradual, incremental abolition would have happened in the Eastern slave states (and maybe Louisiana if the Louisiana Purchase would've been a thing?) completely by c. 1810 - 1830.
 * Due to more immigrant labor being distributed across the country, Labor unionism would've taken off in most of the South at the same time as in the North.
 * More African-Americans moving out westward in Conestoga wagons into Alabama, Mississippi, Kentucky and Tennessee.
 * West Virginia would not have seceded from Virginia the way that it did.
 * The La Amistad case would not have happened as it did.
 * The Atlanta Compromise would not have happened.
 * Sharecropping would not have been a trend.
 * Voting rights would have been extended on a state-by-state basis to African-American men, but with a longer waiting period after the hypothetical last state ends slavery.
 * Interracial marriage and relations would not have been so violently oppressed by the later 19th century.
 * Cities on the Fall Line like Macon, Georgia would not have been established the way that they were because the United States would have continued to recognize the sovereignty of Muscogee and Seminole people via the Treaty of New York (1790).
 * Presuming that someone still invented the phonograph, American music would not have incorporated such influences as jazz (Louisiana) and blues (Mississippi). Popular music - the world over! - would be completely different. Folk spirituals would have been the primary musical expression of African-Americans coming out of slavery in the Eastern states, and African "Congo music" would have stayed very popular for African-Americans coming out of slavery in Louisiana.
 * Jim Crow would not have happened along such a uniform stretch of the country. Instead, petty housing and employment discrimination would've been predominant.
 * Liberia would not have taken shape the way that it did, even though the growth of free Africans in the population (and the Gabriel Prosser slave rebellion of 1800) drove a lot of scared White Americans to the idea that free Africans should follow Jamaican Maroons and other ex-slaves to Sierra Leone.
 * If anything, as White Americans became more hostile to the growth of free Africans (sans the cotton gin) "taking their jobs", and as free African people like Paul Cuffee found prosperity in the shipping business and urban jobs, President Thomas Jefferson would have seen more rationale in supporting deportation of the "excess" of free Africans from the Upper South to Sierra Leone.
 * The War of 1812, for which there were non-slavery-dependent causes, would have seen a larger number of African-Americans escape on British ships for Nova Scotia, Sierra Leone and Trinidad and Tobago.
 * The tobacco-heavy Upper South states (Virginia, Maryland, Delaware, North Carolina) would not have sold over 1 million slaves to plantations in the Lower South (South Carolina, Georgia) nor broken up so many slave (and, like 12 Years a Slave 's Solomon Northrup, free) families in the process. This would result in this region continuing to possess the largest number of African-Americans into the 19th and perhaps 20th centuries.
 * Georgia probably would have abolished slavery after the shortest time of legalized slavery of any of the former 13 colonies (being legalized in GA in 1751).
 * The Black middle class would have grown as the proliferation of education, community organizations (churches, etc.) and jobs filtered further down to the growing number of manumittees.
 * African-American men would have "proven" themselves to the White establishment earlier enough to own property, receive the franchise and pursue local and state elected office.

Other possibilities:
 * Gabriel Prosser, who was literate but enslaved, would have been freed prior to 1800. Hence, his rebellion would not have been planned.