Slime is the Superior Lisp Interaction Mode for Emacs (for CommonLisp).

http://www.common-lisp.net/project/slime/

There's only one release this far, so newer developments are only available from CVS, but what's there is very, very nice. More and more people are using it in favor of IlispMode, given its greater array of features, cleaner code, and more robust design.

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CategoryCommonLisp CategoryEmacs