Martin Heidegger was a phenomenologist, and actively resisted the existentialist label that his contemporaries had desginated for him. His most famous work was Being and Time. This book was hailed as a work of great significance even though (and perhaps because of the fact that) few people, if any, could actually understand it. His life's work centered around the question: "what is being?"
Philosophically, Heidegger's major influences were the pre-socratics, Plato and Aristotle, as well as several 19th century philosophers, most notably Kierkegaard, and Nietzsche.
Today, interest in Heidegger abounds. The conflicting nature of his philosophical thought, his religious life (he had joined a Jesuit noviciate in his early years), his actions as a pro-Hitler member of the Nazi party, and his later denounciation of Hitler have resulted in a seemingly endless stream of books about Heidegger.
Heidegger's influence, once limited to the German and Latin speaking countries (due perhaps to the difficulty of translating his work to English), has since expanded even to the English speaking world.
Britannica.com. "Hedegger Martin", http://www.britannica.com