I am Legend, Duel, The Night Stalker, The Incredible Shrinking Man, almost as many Twilight Episodes as Rod Serling, Night Gallery, Star Trek, A Stir of Echoes, The Prey (Trilogy of Terror) and the list goes on and on.
Impossible to understate his importance to the genre.

I came here knowing that LA and others would have posted on Matheson.
I did not hear of his death until this morning when I read a full page tribute on page 2 of the Cleveland Plain Dealer. It is detailed, hugely respectful and covers not only his work but his influence.
http://www.cleveland.com/tv-blog/index.ssf/2013/06/richard_matheson_legendary_fa...
Not surprising given that the author, the PD television critic, was not just a friend but a colleague, having edited three of Matherson's short story collections.
To add to the detail mentioned by Caleb & LA - - -
Matheson wrote the screenplays of many of the Poe movies, influenced Romero,Stephen King & most other subsequent creators in the genre. His plethora of television and movie scripts practically defined the genre.
Quote: "I don't like the word horror. --- Horror is something visceral, whereas terror is something mental. . . I prefer terror.Horror has become a dirty word. It means blood and guts...body parts. Boris Karloff was a horror actor... Lovectaft a horror writer Now everyone just thinks of the gruesome movie stuff."