Common Lisp (or CL) is an ANSI standardized (1994) language descended from older Lisps such as MACLisp, ZetaLisp, and also from SchemeLanguage and AlgolLanguage. In brief, it is a StronglyTyped and DynamicallyTyped language with LexicalScope (as well as DynamicScope) variables, CallByValue FirstClassFunctions, an extensive type and class system, a large standard library, and a syntax composed out of its own data-structures.
Common Lisp has support for many different kinds of programming styles, and is easily extensible to accomodate new ones.
Misconceptions about Common Lisp : Common Lisp is NOT a purely-functional language, NOT the LambdaCalculus, NOT only for recursion, NOT lacking compilers, NOT 40 years old, NOT abbreviated as "clisp", NOT very similar to SchemeLanguage.
Free CL-on-Unix resources: http://www.cliki.net/
The Common Lisp Hyper Spec: http://www.lispworks.com/reference/HyperSpec/Front/index.htm
Successful Lisp, a tutorial: http://www.psg.com/~dlamkins/sl/cover.html
CL Cookbook: http://cl-cookbook.sourceforge.net/
Association of Lisp Users: http://www.alu.org/ and their CLiki: http://alu.cliki.net/
Available free compilers:
CMUCL: http://www.cons.org/cmucl/
OpenMCL: http://www.clozure.com/openmcl/
GCL and ECL are still incomplete implementations of ANSI CL (GCL is older).
Commercial:
Xanalys Lispworks: http://www.lispworks.com/
Franz Allegro: http://www.franz.com/
Corman Common Lisp: http://www.cormanlisp.com/
Scieneer Common Lisp: http://www.scieneer.com/
Macintosh Common Lisp: http://www.digitool.com/
(Even hobbyists buy software 