Kawa is Scheme on JVM (so is SISC) - a GNU official project.
Runs on Sun and various other JVM's.
Armed Bear Common LISP (ABCL) is CL on JVM
Clojure is new and a bit different -- and sounds very impressive indeed from the presentation(s) at http://clojure.blip.tv
Designed to interoperate easily with Java and other languages on the JVM platform.
The designer (Rich Hickey) has removed dependencies on concrete implementations... e.g. abstracting cons's to first and rest.
seq replaces lists as an abstraction.
It's closer to CL than Scheme (Rich says so).
Looks very promising.