SLIME is an Emacs mode to develop Common Lisp programs. SLIME is an acronym for Superior Lisp Interaction Mode for Emacs. SLIME requires either GNU Emacs or XEmacs, and one of the supported Common Lisp implementations. SLIME itself is free and open source.
Emacs, by itself, can already edit, indent and highlight Common Lisp mode. SLIME integrates Emacs with a Common Lisp implementation. Emacs and SLIME, together, can compile Lisp files, run a read-eval-print loop, show documentation for functions and macros, locate errors in source code, and so on.