Dennis Ritchie is the inventor of the C programming language, which is the ancestor of a whole family of programming languages that includes C++, Java and C# — probably the three most popular programming languages today — as well as D and Objective-C, which are less popular but significant nonetheless.
Ritchie is also one of the authors of the early UNIX kernel, which was the first significant program written in C and for which C was originally designed, and which is the ancestor of a whole family of operating systems that includes Linux, MacOS X, iOS, BSD and many others.
C was the first programming language that allowed the programmer to structure data and code, making it relatively easy to handle very large quantities of data while also maintaining full control of how the hardware is used. Most operating systems today, including significant parts of Windows, are written in C — and most OS designs are at least partly based on UNIX.
Everywhere you look, you can see the fruits of Dennis Ritchie’s labor — and by his fruits you shall know the man — but it seems this man is known only to those of us who either have an intimate knowledge of C and/or UNIX, or are more-than-usually interested in programming language design.
That is a real shame: I think there is a lot we can learn from his legacy — and a lot to be gained from continuing his work.