Steel Bank Common Lisp is a development environment for Common Lisp, with excellent support for the ANSI standard: garbage collection, lexical closures, powerful macros, strong dynamic typing, incremental compilation, and the famous Common Lisp Object System (multimethods and all). It also includes many extensions, such as native threads, socket support, a statistical profiler, programmable streams, and more. These are all available through an integrated, interactive native compiler which feels like an interpreter. SBCL is unique in being a multiplatform native compiler which bootstraps itself completely from source, using a C compiler and any other ANSI Common Lisp implementation.
ACL2 is a mathematical logic, programming language, and mechanical theorem prover based on the applicative subset of Common Lisp. It is an "industrial-strength" version of the NQTHM or Boyer/Moore theorem prover, and has been used for the formal verification of commercial microprocessors, the Java Virtual Machine, interesting algorithms, and so forth.
ClearLisp is a Common LISP interpreter written in C# with the purpose of scripting in a .NET or Mono environment. The ClearLisp language supports a large subset of CL and has an object model with generic functions, class and instance methods, properties, and import of existing .NET classes. ClearLisp executes LISP scripts in interactive mode or in Web mode (IIS; XSP or Apache with Mono). Sample ClearLisp code is provided in the form of a personal, file-based wiki Web application.
CMUCL is a free, high performance implementation of the Common Lisp programming language which runs on most major Unix platforms. It mainly conforms to the ANSI Common Lisp standard. CMUCL provides a sophisticated native code compiler; a powerful foreign function interface; an implementation of CLOS; the Common Lisp Object System; which includes multimethods and a metaobject protocol; a source-level debugger and code profiler; and an Emacs-like editor implemented in Common Lisp. CMUCL is maintained by a team of volunteers collaborating over the Internet, and is mostly in the public domain.
NTW Lisp is a server written in Common Lisp for applications that use the NTW protocol to communicate with a GUI client. This asynchronous protocol makes it possible to serve remote applications that are indistinguishable from native ones. It's also useful for quickly writing GUI apps from Common Lisp that can be run locally.