Famous language designers listed here. Few languages are the work of just one person; but for many languages there is one person predominantly responsible for its design.
People who have designed more than one language or have made a career out of inventing languages:
Niklaus Wirth (see Wirth Languages; Pascal, Algol-W, Modula, Oberon, etc)
Ken Iverson (APL, J)
Guy Steele (Scheme Language, Common Lisp, Java Language, etc.)
Wouter Van Oortmerssen (must hold the record for number of languages designed)
People who have developed languages, in chronological order of their contributions:
RichardPGabriel (Common Lisp)
Guy Steele (Scheme Language, Common Lisp, Java Language -- with others in each case)
Tom Stambaugh, James Gosling (Java Language--though some feel Gosling's technical contributions to Java are overstated)
Yukihiro Matsumoto aka "Matz" (Ruby Language)
Peter Landin (ISWIM)
People who had influence over language design, even if they did not invent a language single-handedly:
a committee (Cobol Language) COBOL was the work of a committee, and should not be credited to Grace Hopper as is usually done; see for instance the IEEE article "The Real Creators of Cobol", March/April 2000 (Vol. 17, No. 2) p30-32, csdl.computer.org 
John Backus, Peter Naur, H. Rutishauer, et al. (Algol Sixty, Fp Language).
Robert Kowalski, his work on automatic theorem proving created the technological underpinnings of Prolog Language.
some of whom eventually become Language Lawyers...
See original on c2.com