He was largely responsible for the existence of England itself. when the Danes invaded britain, they had taken almost all of the land besides a few small marshlands where Alfred based himself, before turning the tide and slowly driving them out of what became England. His work was later continued by his son Edward the Aethling, daughter Aethelflaed and his grandson Athelstan leading to the formation of England.
In addition he also promoted learning and religion throughout the country, insisting that all of his military officers were literate so that he could send written orders - quite revolutionary at the time. although many criticised his piety, there was nought but respect for him and his cunning mind. it is well thought that if Alfred did not do what he did, then we would be living in Daneland now (or some such similar).
I'd recommend reading Bernard Cornwell's books set in that time (the first is called The Last Kingdom), they document the period well, and are also just generally a very good read.