CMU Common Lisp "is a free, high performance implementation of the Common Lisp programming language which runs on most major Unix platforms". It provides an advanced native code compiler, a powerful F...
Common Lisp "on the bare metal."
Steel Bank Common Lisp is a Common Lisp implementation based on the CMUCL sources, but it puts a greater emphasis on maintainability. Most of the SBCL code is in the public domain, the rest under a BS...
A partial implementation of Common Lisp in Common Lisp.