Features
Common lisp allows implementations to have extensions, and has an official way of testing for them, via membership of the *features* list. However, there is no standard for the actual relationship between the name of a feature and its meaning. This page attempts to collect information about currently extant *features* of all Common Lisps.

Feature Meaning
ABCL Present in ABCL
ALLEGRO Present in Allegro Common Lisp
ASDF Added when ASDF-system definition facility is present.
ANSI-CL Implementation purports to conform to the ANSI Common Lisp Standard. In clisp (before 2.26), indicates ansi mode (-a) as opposed to normal mode
CCL Coral Common Lisp. CCL is now Macintosh Common Lisp.
CLISP Present in CLISP
CLTL1 Implementation purports to conform to the first edition of Common Lisp the Language
CLTL2 Implementation purports to conform to the second edition of Common Lisp the Language. Note that according to the letter of the standard :CLTL2 "must not be present in any conforming implementation"
CMU Indicates that the lisp is CMUCL
CMU17 The lisp is CMUCL, of version 18b or earlier
CMU18 The lisp is CMUCL, version 18a or later (FIXME: What versions of Peter's correspond to which)
COMMON-LISP Should be present in any vaguely Common Lispy implementation
DRAFT-ANSI-CL Implementation purports to conform to the first draft of ANSI CL
IEEE-FLOATING-POINT Implementation purports to conform to the IEEE specification for floating point arithmetic
INFIX Feature added when the #i infix reader macro is available.
INTERFACE-DESIGNER MCL interface toolkit
MK-DEFSYSTEM Feature added when Mark Kantrowitz' defsystem is present
PARENSCRIPT Added by Parenscript Lisp to JavaScript translator to indicate that other libraries may define Parenscript-specific macro duals to enable their translation to JavaScript (see uri-template for an example).
PCL In CMUCL, indicates that CLOS (the Portable Common Loops version) is present
POPLOG A feature found identifying the Poplog Lisp
SBCL Present in Steel Bank Common Lisp
UNICODE In CLISP, indicates that characters are internally unicode
X3J13 Implementation conforms to some draft of ANSI CL
LISPM Lisp Machine (eg. used in Moon's MapForms sources).
EXPLORER Ti Explorer (eg. used in Moon's MapForms sources).

Implementation *FEATURES*
CMUCL (:PCL-STRUCTURES :PORTABLE-COMMONLOOPS :PCL :PYTHON :SMALL :NO-HEMLOCK :NO-CLM :NO-CLX :GENCGC :CONSTRAIN-FLOAT-TYPE :PROPAGATE-FUN-TYPE :PROPAGATE-FLOAT-TYPE :RANDOM-MT19937 :MP :HASH-NEW :COMPLEX-FLOAT :SIGNED-ARRAY :CMU18 :DIRECT-SYSCALL :RELATIVE-PACKAGE-NAMES :X86 :LINUX :GLIBC2 :UNIX :COMMON :CMU :NEW-COMPILER :CLTL2 :COMMON-LISP :ANSI-CL :DRAFT-ANSI-CL :X3J13 :IEEE-FLOATING-POINT)
CLISP
Documentation on *features*
(:CLOS :LOOP :COMPILER :CLISP :ANSI-CL :COMMON-LISP :INTERPRETER :LOGICAL-PATHNAMES :FFI :GETTEXT :UNICODE :BASE-CHAR=CHARACTER :UNIX)
pre 2.26 had :CLTL2 instead of :ANSI-CL, except when using -a/--ansi switch
Depending on host system: one of :UNIX :WIN32 :AMIGA :OS/2 :ACORN-RISCOS
:PC386 is there with :UNIX Linux Intel/AMD, :WIN32 or :OS/2 systems.
:FFI, :UNICODE, :SCREEN, :SOCKETS :LOGICAL-PATHNAMES are present on most systems CLISP runs on, but are in fact compilation choices, as well as :GETTEXT, :SYSCALLS or :DIR-KEY.
SCL 1.1.1 trial on Linux (:MK-DEFSYSTEM :COMPILER :IEEE-FLOATING-POINT :ANSI-CL :COMMON-LISP :SCL :COMMON :UNIX :GLIBC2 :LINUX :X86 :I486 :DIRECT-SYSCALL :GENCGC :PROPAGATE-FUN-TYPE :PROPAGATE-FLOAT-TYPE :CONSTRAIN-FLOAT-TYPE :CONSERVATIVE-FLOAT-TYPE :HASH-NEW :RANDOM-MT19937 :RELATIVE-PACKAGE-NAMES :PTHREAD :SMP)
ECL ECoLisp 0.0g : (:DEFPACKAGE :LOOP :IEEE-FLOATING-POINT :IBM-PC :I386 :BSD :UNIX :ANSI-CL :CLOS :ECL :COMMON)
ECL 0.8, Mac OS X: (:IEEE-FLOATING-POINT :APPLE :POWERPC :BSD :UNIX :ANSI-CL :CLOS :BOEHM-GC :ECL :COMMON)
ECL 0.9, Linux: (:NEW :IEEE-FLOATING-POINT :BSD :UNIX :DLOPEN :CLOS :BOEHM-GC :ECL :COMMON :ANSI-CL :COMMON-LISP :I386 :IBM-PC)
GCL (:COMPILER :NUMLIB :SDEBUG :DEFPACKAGE TRUNCATE_USE_C CLX-LITTLE-ENDIAN BSD
MC68020 BSD386 SPARC (arch-dependent)
SGC IEEE-FLOATING-POINT UNIX GMP GCL AKCL COMMON KCL)
Symbolics Genera 8.3 (:POSTSCRIPT-CLIM :CLX-CLIM :GENERA-CLIM :CLIM-EXTENDS-CLOS :CLIM-ANSI-CONDITIONS :CLIM-USES-LISP-STREAM-FUNCTIONS :DEFSYSTEM :ANSI-90 :GENERA-RELEASE-8-3 :GENERA-RELEASE-8 :SILICA :CLIM-2.0 :CLIM-2 :CLIM :DEFSTORAGE :CLX-MIT-R5 :CLX-MIT-R4 :XLIB :CLX :CLX-OVERLAPPING-ARRAYS :CLX-LITTLE-ENDIAN :CLX-ANSI-COMMON-LISP :LOOP-PREFER-POP :LOOP :DEFSTRUCT :DEBUG-SCHEDULER-QUEUES :NEW-SCHEDULER :LISPM :SYMBOLICS :GENERA :ROW-MAJOR :IMACH :CHAOS :IEEE-FLOATING-POINT :SORT :FASLOAD :STRING :NEWIO :ROMAN :TRACE :GRINDEF :GRIND)
MCL 5.0b (:ANSI-CL :ANSI-MAKE-LOAD-FORM :NEW-TABLES :PRIMARY-CLASSES :CCL :CCL-2 :CCL-3 :CORAL :APPLE :DIGITOOL :COMMON-LISP :MCL :CLTL2 :PROCESSES :CARBON-COMPAT :INTERFACES-2 :CCL-4 :CCL-4.2 :CCL-4.3 :CCL-4.3.1 :CCL-4.4 :CCL-4.3.5 :CCL-5.0 :POWERPC :PPC-TARGET :PPC-CLOS)
OpenMCL (:PRIMARY-CLASSES :CCL :CCL-2 :CCL-3 :CCL-4 :CORAL :DIGITOOL :COMMON-LISP :MCL :OPENMCL :ANSI-CL :PROCESSES :INTERFACES-2 :POWERPC :PPC-TARGET :PPC-CLOS :CLOZURE :DARWINPPC-TARGET :DARWINPPC-HOST :DARWIN :POWEROPEN-TARGET)
SBCL (:LINUX (os-dependent) :ALPHA (arch-dependent) :IEEE-FLOATING-POINT :SB-TEST :SB-DOC :UNIX :SBCL :COMMON-LISP :ANSI-CL)
The meaning of most SBCL features is documented in base-target-features.lisp-expr in the source archive
Corman Lisp
(Win32)
(:CORMANLISP :PL :COMMON-LISP :WIN32)
Allegro CL 6.0
(Linux x86)
(:ACL-SOCKET :HIPER-SOCKET :PROFILER :MULTIPROCESSING :FLAVORS :LITTLE-ENDIAN :GSGC :COMPILER :USE-STRUCTS-IN-COMPILER :CLOS :DYNLOAD :DLFCN :UNIX :LINUX :REDHAT6 :LINUX86 :X86 :VERIFY-CAR-CDR :RELATIVE-PACKAGE-NAMES :MODULE-VERSIONS :IEEE :IEEE-FLOATING-POINT :CONFORMING-IEEE :COMMON-LISP :ANSI-CL :DRAFT-ANSI-CL-2 :X3J13 :ALLEGRO :EXCL :FRANZ-INC :ALLEGRO-VERSION>= :DYNLOAD-ACL-LIBRARY :ALLEGRO-V6.0)
Allegro CL 6.1
(Linux x86)
(:ACL-SOCKET :HIPER-SOCKET :PROFILER :MULTIPROCESSING :FLAVORS :LITTLE-ENDIAN :GSGC :COMPILER :USE-STRUCTS-IN-COMPILER :CLOS :DYNLOAD :DLFCN :UNIX :LINUX :REDHAT6 :LINUX86 :X86 :VERIFY-CAR-CDR :ENCAPSULATING-EFS :RELATIVE-PACKAGE-NAMES :MODULE-VERSIONS :IEEE :IEEE-FLOATING-POINT :CONFORMING-IEEE :ICS :COMMON-LISP :ANSI-CL :DRAFT-ANSI-CL-2 :X3J13 :ALLEGRO :EXCL :FRANZ-INC :ALLEGRO-VERSION>= :DYNLOAD-ACL-LIBRARY :ALLEGRO-V6.1)
Allegro CL 6.2 (Linux x86) (:ALLEGRO-CL-ENTERPRISE :ACL-SOCKET :HIPER-SOCKET :PROFILER :MULTIPROCESSING :FLAVORS :LITTLE-ENDIAN :GSGC :COMPILER :USE-STRUCTS-IN-COMPILER :CLOS :DYNLOAD :DLFCN :UNIX :LINUX :REDHAT6 :LINUX86 :X86 :VERIFY-CAR-CDR :ENCAPSULATING-EFS :RELATIVE-PACKAGE-NAMES :MODULE-VERSIONS :IEEE :IEEE-FLOATING-POINT :CONFORMING-IEEE :ICS :COMMON-LISP :ANSI-CL :DRAFT-ANSI-CL-2 :X3J13 :ALLEGRO :EXCL :FRANZ-INC :ALLEGRO-VERSION>= :SYMBOL-VALUE-VECTOR :DYNLOAD-ACL-LIBRARY :ALLEGRO-V6.2)
Allegro CL 6.2 Windows (:ALLEGRO-CL-ENTERPRISE :ACL-SOCKET :HIPER-SOCKET :PROFILER :MULTIPROCESSING :FLAVORS :LITTLE-ENDIAN :GSGC :COMPILER :USE-STRUCTS-IN-COMPILER :CLOS :VERIFY-CAR-CDR :DYNLOAD :DLWIN :X86 :MSWINDOWS :MICROSOFT :MICROSOFT-32 :ENCAPSULATING-EFS :RELATIVE-PACKAGE-NAMES :MODULE-VERSIONS :IEEE :IEEE-FLOATING-POINT :CONFORMING-IEEE :ICS :COMMON-LISP :ANSI-CL :DRAFT-ANSI-CL-2 :X3J13 :ALLEGRO :EXCL :FRANZ-INC :ALLEGRO-VERSION>= :OS-THREADS :TESTWINAPI :SYMBOL-VALUE-VECTOR :DYNLOAD-ACL-LIBRARY :ALLEGRO-V6.2)
Poplog 15.53 clisp
(unrelated to other clisp)
(:POPLOG :COMMON :CLOS :PC 80386 :UNIX :LINUX 2.0 :ELF :POSIX)
LispWorks for Windows, Enterprise Edition (:DBCS-ENV :COMPILE-REVERSE :BYTE-INSTRUCTIONS HARP::PC386 :COMMON-LISPWORKS :COMMON-DEFSYSTEM :CAPI-TOOLKIT :CAPI :LW-EDITOR :COMMON-FFI :NEW-PATCH-SYSTEM :DBCS :UNICODE :WIN32 :IAPX386 :FLOAT :CLOS :ANSI-CL :COMMON-LISP :LATIN-1 :LISPWORKS :LISPWORKS4.2 :LISPWORKS4 :HARLEQUIN-COMMON-LISP :HARLEQUIN-PC-LISP :NEW-PC-COMPILER :COMPILER :REVERSE :LITTLE-ENDIAN :CLASS-SHAKE-USING-GATES :SUPPORTS-CONS-BLOCKS :IEEE-FLOATING-POINT)
LispWorks for Macintosh, Enterprise Edition Darwin/Motif image:
(:COMMON-LISPWORKS :LW-EDITOR :CAPI-XM-LIB :CAPI-TOOLKIT :CAPI :X11-MOTIF :FFI-X11-XM :FFI-X11-XT :FFI-X11-XLIB :FFI-X11 :X11-MOTIF-NO-LIBRARIES :DBCS-ENV :COMMON-FFI :NEW-PATCH-SYSTEM HARP::POWERPC :BIGIT16-DIV :LL-GLINK :FLUSH-BY-ADDRESS :FOREIGN-ALIGNED :SPLIT-WORD-INSTRUCTIONS :WORD-INSTRUCTIONS :COMPILER :SHALLOW-BINDING :ANSI-CL :COMMON-LISP :IEEE-FLOATING-POINT :LISPWORKS :CLASS-SHAKE-USING-GATES :COMMON-DEFSYSTEM :CLOS :DBCS :UNICODE :UNIX :HARLEQUIN-COMMON-LISP :HARLEQUIN-UNIX-LISP :LATIN-1 :LISPWORKS4 :LISPWORKS4.3 :DARWIN :MAC :MACOSX :APPLE :RS6K HARP::RS6K :NTS :HAVE-SHARED-LIBRARY :MAPPED-IMAGE :FLI-REGISTER-MODULE :PTHREADS)
Darwin/Cocoa image:
(:COMMON-LISPWORKS :LW-EDITOR :CAPI-COCOA-LIB :CAPI-TOOLKIT :CAPI :DBCS-ENV :COCOA :UNIX-WITHOUT-MOTIF :COMMON-FFI :NEW-PATCH-SYSTEM HARP::POWERPC :BIGIT16-DIV :LL-GLINK :FLUSH-BY-ADDRESS :FOREIGN-ALIGNED :SPLIT-WORD-INSTRUCTIONS :WORD-INSTRUCTIONS :COMPILER :SHALLOW-BINDING :ANSI-CL :COMMON-LISP :IEEE-FLOATING-POINT :LISPWORKS :CLASS-SHAKE-USING-GATES :COMMON-DEFSYSTEM :CLOS :DBCS :UNICODE :UNIX :HARLEQUIN-COMMON-LISP :HARLEQUIN-UNIX-LISP :LATIN-1 :LISPWORKS4 :LISPWORKS4.3 :DARWIN :MAC :MACOSX :APPLE :RS6K HARP::RS6K :NTS :HAVE-SHARED-LIBRARY :MAPPED-IMAGE :FLI-REGISTER-MODULE :PTHREADS)