Benjamin Banneker was a free black man in the late 1700's and early 1800's in Maryland. As a mathemetician and surveyer, he was the first to map out and survey the Washington, DC area. His almanac, "Benjamin Banneker's Almanac," was published during the 1790's and was popular in Maryland, Virginia and points westward, into Kentucky.
If I recall correctly, he spent most of his life in and around Ellicot City, MD, near Baltimore.
Err, well, like so many things in life, this one is only sort of true. The initial surveying and mapping of the District of Columbia was done by a fellow by the name of Andrew Ellicott, who had been appointed to the task by Washington himself - Banneker was Ellicott's assistant at the time, which is probably where much of the confusion arises.