Resin
Java based XSL focused on dynamic server pages. This Java servlet supports JavaScript and JavaServer Pages (JSP). It also compiles JavaScript templates into Java byte code. [Commercial]

XTask
XSLT optional task for the Java build tool Ant. Uses XT. [Open Source]

4XT
A site of resources for XT (a Java XSLT processor), by and for XT users, powered by XT. Includes news, mail list, docs, FAQ, links.

Xalan-Java
Provides high-performance XSL stylesheet processing. [Open Source, BSD-like]

XSLTC
A compiler from XSLT to Java bytecode. [Open Source, BSD-like]

Lotus XSL Processor
Java-based XSLT Processor, based on Xalan. [Freeware]

A Better Way for Web Development
This article discusses Cocoon, a Java servlet that enables the use of XSLT and XML to separate logic and layout during Web development.

SAXON
An XSLT processor, and a Java library allowing full programming capability using a similar processing model. [Open Source, MPL]

XT
An implementation in Java of XSL Transformations (XSLT). [Open Source]