Being provoked into trying to learn a bit more about CLOS's method of multiple dispatch, and wanting to find an implementation, led me on this search path in a few spare minutes:
1. Search Google for CLOS
2. Click http://www.dreamsongs.com/CLOS.html, which is on the site of Richard Gabriel ("worse is better")
3. Click the intriguing title CLOS: Integrating Object-Oriented and Functional Programming
4. Read with interest on the first page the essence of multi methods:
A generic function, or polymorphic function as it is sometimes called, is one whose implementation depends on the types of its arguments. That is, the code that is executed for a particular set of arguments is mechanically determined by the types of the arguments.
In strictly functional languages, an operation is defined by a single, monolithic piece of code; any argument-type conditionality is expressed as code explicitly programmed in by the user. In contrast, the CLOS notion of generic functions supports automatic selection from among separately defined, typespecific implementational parts. Yet, from the client’s point of view, generic functions and traditional ordinary functions are called in the same way: The procedural abstraction barrier is still in force.
5. Start looking for implementations. Search reddit for "lisp implementations".
6. Click Common Lisp Implementations: A Survey.
7. The survey lists 11 Common Lisp implementations. Of these, only 5 are listed as available on the Unixes and Windows. Of those, only 3 do not have commercial licenses. Two of the remaining choices have weird names: Armed Bear Common Lisp, which does not look promising (its own home page says, under "Bugs", that "ABCL's CLOS is intolerably slow"), and Embedded Common Lisp (ECL). It's difficult to tell what immediately from Wikipedia or Google what kind of support has. But the remaining candidate, GNU clisp, appears to be in good standing, judging from the Wikipedia article (where it is claimed this is the platform Paul Graham used for Viaweb), and the Sourceforge stats which claim 100 downloads a day in the past week.
8. Find the Clisp site. Another Lisp site without an obvious Download link.