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 (Or would, if it weren't 404 See document at archive.org). It's from James Arvo's Caltech CS class.
- A Brief Guide to CLOS by Jeff Dalton.
- Tae-Sun Chung's CLOS Overview (Or would've been one, if it weren't 404. archive://)
- CLOS-DB is a database interface for the Common Lisp Object System.