Dennis was coauthor of the original UNIX operating system and author of the first implementation of the C language. This is to say, the majority of modern computing builds directly on his work, and most of the rest was directly influenced by it.
One of his collaborators, Rob Pike, shared this message on Google Plus:
As Dennis's siblings, Lynn, John, and Bill Ritchie--on behalf of the entire Ritchie family--we wanted to convey to all of you how deeply moved, astonished, and appreciative we are of the loving tributes to Dennis that we have been reading. We can confirm what we keep hearing again and again: Dennis was an unfailingly kind, sweet, unassuming, and generous brother--and of course a complete geek. He had a hilariously dry sense of humor, and a keen appreciation for life's absurdities--though his world view was entirely devoid of cynicism or mean-spiritedness.
We are terribly sad to have lost him, but touched beyond words to realize what a mark he made on the world, and how well his gentle personality--beyond his accomplishments--seems to be understood.