The following resources are the ten best available free online for African-American genealogy, in no particular order:
1. DISTRICT OF COLUMBIA: List of Owners Who Filed Petitions  (District of Columbia Archives)
When the U. S. Congress abolished slavery in the District of Columbia in 1862, a provision in the law allowed slave owners to petition the courts for compensation for their slaves that were freed. This page presents digital images of the register of petitions, containing the full names of both slave owner and each slave being claimed, as well as other information relating to each claim.
2. GEORGIA: Marriage Records from Microfilm (Georgia Archives)
This site presents digital images of most of the extant county marriage registers for the state of Georgia. This includes the separate "Colored" registers created following the Civil War. In some counties, the names of slave owners were recorded with the marriages of former slaves.
3. CALIFORNIA: Slavery Era Insurance Registry (California Department of Insurance)
In 2000, the state of California enacted a statute that required all insurance companies conducting business within the state to "research and report to the Commissioner with respect to any records within the insurer's possession or knowledge relating to insurance policies issued to slaveholders that provided coverage for damage to or death of their slaves." This registry contains the Department of Insurance's reports to the California legislature on the information obtained, as well as a database of the names of the insuring slave owners and the insured slaves. Though collected by California, the database contains information on slaves and slave owners nationwide.
4. MARYLAND: Prince George's County Slave Statistics (Maryland State Archives)
and, Montgomery County Slave Statistics
In 1867 the Maryland General Assembly created a new county office called the Commissioner of Slave Statistics. The Commissioner's purpose was to collect and compile registers of the names, ages, and other information concerning every slave owned within the state at the time of the abolition of slavery in 1864, for purposes of petitioning the federal government for compensation (as had previously been done in the District of Columbia). Though the registers no longer exist for all of the counties, several counties do still exist, though never microfilmed. The above sites contain digital images of the registers for Prince George's and Montgomery counties.
5. MISSISSIPPI: Freedmen's Bureau Search (Mississippi Department of Archives and History)
No single record group contains more information on African-Americans following the Civil War than the records of the field offices of the Bureau of Refugees, Freedmen, and Abandoned Lands (the "Freedmen's Bureau"). Yet very few indexes of these records exist. This database contains a searchable index to work contracts recorded by the Freedmen's Bureau in Mississippi.
6. SOUTH CAROLINA: Lowcountry Africana
No single website contains more information on former slaves in South Carolina. Among the resources digitized here are volumes of Freedmen's Bureau records (primarily work contracts), 1868 voter registration records, and various plantation records. The site also has partnered with Footnote.com to offer free images of all of the Charleston estate inventories.
7. MISSOURI: Civil War Slave Compensation Claims From Compiled Military Service Records of U.S. Colored Troops (St. Louis County Public Library)
In the four loyal slave states during the Civil War (Missouri, Kentucky, Maryland, and Delaware), loyal slave owners whose slaves enlisted in the U. S. Colored Troops were eligible for compensation for these slaves. This website provides a database of all of these claims granted within the state of Missouri.
8. VIRGINIA: Cohabitation Registers (Library of Virginia)
These registers recorded the names of formerly enslaved couples that had been living together as man and wife. Under Virginia law, marriages between slaves were not legally binding, but these registers were a first attempt to recognize these marriages. The site contains digital images of the cohabitation registers, as well as registers of the "children of colored persons whose parents had ceased to cohabit."
9. NATIONWIDE: Freedmans Bank Records, 1865-1874 (FamilySearch.org)
Following the Civil War, the U. S. Congress chartered the Freedman's Savings and Trust Company (the "Freedman's Bank") in order to assist former slaves to become financially independent. These account slips for over 67,000 African-Americans during this period can contain varying amounts of information, including the names of former slave owners or plantations, and the names of spouses, parents, children, and siblings. Not all slips contain exactly the same information, but the collection as a whole is one of the most valuable private record groups for African-American research.
* The record groups on this site do not have static, dedicated URLs. To locate this record set, browse through the records available for the United States.
10. MISSOURI: [St. Louis] Freedom Suits Case Files, 1814-1860 (Missouri State Archives)
Under Missouri law between 1814 and 1860, slaves could petition the courts for their freedom. Digital images of these case files appear on this website, including the case of the slave Dred Scott. The Dred Scott later became a landmark U. S. Supreme Court decision regarding slavery, shortly before the beginning of the Civil War.