Hugh Capet was a nobleman who was elected the first King of the Franks (Rex Francorum) after the Carolingian Dynasty. His line continued to rule the area that would become France for the next 400 years, and the cadet branches of his house, Valois and Bourbon, ruled France until the revolution.
What made him so unique, and his legacy so enduring?
Hugh Capet wasn't that successful in his lifetime, but to no fault of his own. His room to maneuver was extremely limited by his personal lack of power and his unruly vassals. There are a few reasons he is famous:
As count of Paris, the royal demesne [royal lands] became centered around Paris during his lifetime. This area, known as the Ille-de-France, remained the center of French royal power during the rest of the medieval ages and it later became the capital, the cultural center, and the economic center of the country. The Kingdom of France was initially very weak [some of the later Carolingian kings of "France" had a royal demesne limited to a single castle] and even more decentralized than the Holy Roman Empire, so Hugh Capet is important in that he gave his successors a firm base to work and expand from.
He was the first king in a long line of very successful monarchs that eventually came to be the most powerful rulers in Europe. The fact that he was the first of the Capetian kings makes him notable. France initially was basically an elective monarchy, and the Capetians were able to make it primogeniture [oldest son inherits].
In short, Hugh Capet is famous not for his accomplishments but for the accomplishments of his successors. The Capetians were able to turn a very decentralized monarchy [even by the standards of the time, the French kings had very little power] into arguably the most centralized monarchy in Europe by the Renaissance period.