Any purporting-to-conform ANSI CL implementation will have CLOS built in. There are also "add-on" implementations from before it was part of standard CL: Closette is the code developed in the AMOP book and probably not practically useful, whereas PCL is fully-featured (if slower than a typical tuned commercial implementation might be) and has been used as the basis for the CLOS support in e.g. CMUCL and SBCL
- Marty Hall's Introduction to the Common Lisp Object System can be found here
- This page describes the bare essentials of CLOS. It's from James Arvo's Caltech CS class.
- A Brief Guide to CLOS by Jeff Dalton.
- Tae-Sun Chung's CLOS Overview
- CLOS overview and concept papers on Richard P. Gabriel's site.
- CLOS-DB is a database interface for the Common Lisp Object System.
- defclass-std is a shortcut macro to write DEFCLASS forms quickly (in Quicklisp). For example, the following
It is very flexible. The repository contains examples from other projects (Clack, etc).