Anyone know if there have been attempts at concurrent garbage collection in any of the major lisp implementations (whether CL or Scheme). Maybe Clojure?
Clojure uses the underlying host's GC (e.g., JVM, CLR or JS engine). It also typically generates rather more garbage than other languages may due to immutability by default. As with all things, you can avoid the garbage if you are willing to abandon the usual idioms.