Lisp began as a language development project of John McCarthy. He has documented its earliest history in his papers "LISP---NOTES ON ITS PAST AND FUTURE---1980" and "History of Lisp". These two papers were composed by McCarthy working from his own memory. However he has noted that Herbert Stoyan's review is probably more accurate. For other references there is also the excellent "Evolution of Lisp" by Gabriel and Steele which covers the 'golden age' of Lisp development, and Pitman and Miller's "Brief History of Lisp". The old C.L.L FAQ also has a timeline through 1992. See also the ALU page on the history of Lisp.
CMUCL comes to us from Spice Lisp, a Lisp implementation at Carnegie-Mellon University on the CMU-designed PERQ workstation. Hemlock of course also descends from the editor of the same name implemented in Spice Lisp. Spice Lisp was notable for being continually updated in step with the developing Common Lisp standard through the 1980s. At some point it was recognized that it had become a Common Lisp and was renamed to its present moniker. SBCL is a recent fork from CMUCL.
GCL and ECL have roots in the first 'outsider' Common Lisp, Kyoto Common Lisp (KCL) implemented by Taiichi Yuasa and Masami Hagami based on their reading of the CLtL1. That implementation pointed out a lot of defects in the Common Lisp specification which had been either assumed or unnoticed by close community effects among the standardizers. KCL gave birth to the KCL Family, in particular Austin Kyoto Common Lisp (AKCL), developed from KCL by William Schelter at the University of Texas, Austin. His aim was to provide a Lisp implementation which would run Maxima, the descendent of the MIT MACSYMA symbolic algebra system. Schelter later released AKCL as GCL under the GPL. (What about ECL?)
OpenMCL is an open source Common Lisp implementation, licensed under a Lisp-specific variant of the LGPL, and derived from Digitool's MCL product. Versions are available for LinuxPPC, Linux 64-bit x86, and for Darwin (PPC and x86-64 bit), the BSD/Mach layer on which Mac OS X is based.
Its history is an interesting path from commercial, to important government research, and finally to an open source project. It encompasses the highs of technical achievement, the lows of internecine battles over implementation language, and finally the charitable release of the software as open source.
More details can be found on the Clozure site.
Add links to other historical information sources here...
There is a really nice set of emails from the early 80s when Common Lisp was being developed here.