Hans Christian Ørsted (1777-1851) was a physicist, who discovered electromagnetism and the speed of light.
Hans Christian Andersen (1805-1875) was an author, who wrote a bunch of famous fairytales (The little mermaid, The ugly duckling, The emperor's new clothes etc.).
Niels Bohr (1885-1962) was a physicist, who made foundational contributions to understanding atomic structure and quantum theory. Involved in the Manhattan Project during WW2.
Bjarne Stroustrup (1950-) is a computer programmer who invented the programming language C++.