CMUCL was developed as "open source" (Publich Domain). It was a reworked Spice Lisp. Several other implementations used code from there. Additionally there was an open source LOOP, an open source CLOS implementation, etc.
As former maintainer of ECL, another open software Common-Lisp, I can attest that both CMUCL's and SBCL's are extremely useful as reference implementation for much of ANSI CL. I learned a lot regarding LOOP, or how to implement the floating point printer, which has a lot of complexity (and reproducibility) as compared to ANSI C and other standards.