My great grandfather, Milton Mills Brooks, and his twin brother, Milford, were adopted by the Brooks family before they were three years old. Their older half-sister, Julia Morehouse, was also taken in by the Brooks family, but later adopted by the Haskell family.
Milford Brooks was returned to the county poorhouse as a child, and later resided with his mother and older half-sister. On the 1880 census, his surname is shown as Rice, which is the surname of his biological father.
I learned about the adoption of Milton and Milford after my dad died in 1982, and I would say this particular event really drew me into genealogy.
It took me about ten years to identify the birth mother: Mary Rowley. She had been married to John Morehouse and had three daughters, but then John died.
After nearly forty years, my family and I have come pretty close to confirming the birth father as Anson T. Rice, who was in the right place at the right time - but was a married man with eight or nine children. Anson and his wife separated in the time period.
Two of Milton's male descendants, and two of Anson's male descendants have done the 111-marker Y-chromosome test, and are close matches. I am hoping we can upgrade to the Big Y-700 for all of them.
Lastly, other branches of my family have adopted children as well - many of which I have been able to identify.