SISC Scheme is a very complete Scheme interpreter and compiler written in Java. Particularly interesting: there is a continuation-based web framework for it.
Other interesting features include good integration into the Java world through the Java-Scheme interface. In principle, all libraries from the Java world are also available in Scheme.
SISC Scheme also supports SLIB (an extension library for Scheme with many useful modules) and various SRFIs (Scheme Requests for Implementation - the formal way to extend the Scheme language with standard modules).