Mark Williams Let's C 4.0.12
Released in 1987 by Mark Williams Company
For DOS

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The Mark Williams Let's C compiler was powerful C compiler for
MS-DOS. Let's C was a subset of the Mark Williams C Programming
System, and the first low cost entry-level professional C compiler
for the IBM PC. It was also the first compiler to have a C
source-level debugger.

It boasts the ability to create faster, smaller, and better optimized 
executables than any other compiler on the market at the time. It
supports both large and small x86 models, and supports the 8087 math
co-processor. It competed against compilers from Digital Research,
Lattice, Computer Innovations, and Microsoft.

It includes a complete implementation of Kernighan and Ritchie C
language, plus extensions to C implemented under UNIX. Includes an
assembler, loader, archiver, advanced symbolic debugger. C shell,
utilities and Micro-EMACS, a full-screen editor.

There were versions of the Mark Williams C Programming System for
Unix and the Atari ST. Let's C is fully compatible, so C programs can
easily be ported between the ST and IBM PC.

The Mark Williams Company was also notable as having produced 
Coherent, the first commercially viable Unix clone.