He convinced Congress to purchase Alaska.
William H. Seward always was a little bit ahead of his time. He embraced anti-slavery policies before they were popular (He was born on a farm in New York that used slave labor). He ran against Abraham Lincoln for the Republican presidential nomination in 1860; As the Secretary of State for Lincoln (and later, Andrew Johnson), he kept England and France from getting directly involved with the Civil War.
He is most well-known for persuading Johnson and the US Senate to purchase Alaska from the Russian Empire. Most people looked at it on the map and assumed it was a frozen wasteland, but in an era when America's westward expansion seemed to end at the Pacific Ocean, Seward saw potential for expanding into Alaska, Hawaii and Samoa.