The era when Lisp had its greatest commercial success (the 1980s) was a time when free (as in either beer or speech) high-quality development tools for any language were rare. This was the golden age of proprietary software; people were expected to pay for operating systems, compilers, editors, and other tools, and there was increasingly no expectation of having access to the source code. The GNU project was started in 1983 by Richard Stallman, who used to work on MIT Lisp projects (many of his colleagues became part of either Lisp Machines, Inc. or Symbolics, which all came from MIT’s work on Lisp machines).

Back to proprietary software, during the AI boom of the 1980s, Lisp machine vendors had success selling high-end workstations to customers willing to pay good money for Lisp environments. This dried up during the subsequent AI winter, though some customers were able to move their Common Lisp solutions to commercial Lisp implementations that worked on workstations or servers running Unix or Windows. But at this point Lisp no longer had the same level of commercial interest, though the legacy and mindshare of Lisp grew through the use of Scheme in CS education (e.g., SICP and the team that wrote PLT Scheme, which was renamed Racket) and the advocacy of Lisp from prominent developers and researchers like Paul Graham, Richard Stallman (Emacs), Eric S. Raymond, Alan Kay (while he’s of course a Smalltalker, he’s spoken fondly of LISP 1.5 and also of The Art of the Metaobject Protocol), and many others. There are also many people who used Symbolics Genera in particular and who speak highly of its development environment, sometimes expressing the sentiment that modern development environments don’t compare to it.

There are FOSS Common Lisp implementations with wide usage. The most notable is Steel Bank Common Lisp (SBCL), and I also hear of plenty of people using Armed Bear Common Lisp (ABCL, which runs on the Java virtual machine) and Embeddable Common Lisp (ECL). There are other FOSS Common Lisp implementations that I know less about.