A Common Lisp implementation with an IDE provided by LispWorks Ltd (formerly by Xanalys pre 2005, and formerly by Harlequin pre 2000).  Provides many features to speed productivity when writing Lisp code, including class/function/GenericFunction browsers, an interface builder, a debugger, source code stepper, an inspector, and a REPL.  Website is here:

http://www.lispworks.com

A free "Personal Edition" is available for download for non-commercial and evaluation purposes.  It has run time and heap-size limits, and does not support the creation of standalone executables.

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I've found the Lesstif toolkit that the Linux Personal Edition uses to be full of hassles.  It's buggy, inconsistent, and doesn't offer a native look and feel.  Is there any way to get it to use the native (KDE) widget set?  -- JonathanTang

''KDE isn't possible (or really native), but try Open Motif 2.2 for fewer bugs.''

Update: LispWorks 6.0 for Linux uses GTK+ instead of OpenMotif/Lesstif. -- Dave Fox, LispWorks Ltd.