FamilySearch has digitized the volume Register of Negroes and Mulattoes, 1800-1820, Adams County, PA. You can access the volume via the FamilySearch catalog here.
This register, remarkably, lists exact birthdates for many of those whose names appear. Information in the register includes name, occupation and residence of the enslaver and name, gender, birthdate and date arrived for enslaved people.
Sample Image

What treasures have you found in the FamilySearch catalog? Let us know in the comments below!