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.
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.
CLSQL is an SQL database interface for Common Lisp. It provides object-oriented and functional access methods to the underlying database, which can be one of MySQL, ODBC, PostgreSQL, or SQLite. It uses the Unified Foreign Function Interface (UFFI) and thus supports the CMU Common Lisp, Steel Bank Common Lisp, and Allegro Common Lisp implementations.
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.
SLIME is an integrated development environment for Common LISP which does everything you would expect from an IDE: code evaluation, compilation, macro expansion, and auto-completion. It also finds definitions of functions, and marks LISP forms which the compiler finds to be erroneous. It provides easy access to implementation-specific online documentation as well as the ability to look up symbols in the ANSI Common Lisp HyperSpec. Further, it includes an interactive debugger and object inspector.
STMX is a high-performance Common Lisp library for composable Transactional Memory (TM), a concurrency control mechanism aimed at making concurrent programming easier to write and understand. Instead of traditional lock-based programming, one programs with atomic memory transactions: if a memory transaction returns normally it is committed. If it signals an error, it is rolled back. Transactions can safely run in parallel in different threads, are re-executed from the beginning in case of conflicts or if consistent reads cannot be guaranteed, and effects of a transaction are not visible from other threads until committed. This gives freedom from deadlocks, automatic rollback on failure, and aims to resolve the tension between granularity and concurrency.