There's also SISC, which purports to be a full implementation of R5RS Scheme for the JVM.
Are you also familiar with Clojure? How would you compare the two?
I'm not familiar with Clojure, but as I understand it's a new language. Kawa is R5RS Scheme except it can't capture continuations. Kawa has the advantage of being a fast compiler. SISC is an interpreter, but it is more completely standards-compliant Scheme.
http://ourdoings.com/2004-08-24