XML Binary Infoset Encoding (XBIS) is an encoding designed to eliminate most of the padding of XML text documents being passed between programs, while being faster to generate and interpret. The focus is more on speed than on size, so if document size is the major concern standard compression algorithms can offer superior results. The current Java implementation shows 4-8X performance benefits over standard XML parsers over a range of document types and sizes and across JVMs tested.
JaxMeXS is a parser for XML Schema, written in Java. Unlike Xerces or similar parsers, this one isn't written for validation of conforming instances, but for schema authors who like to use the schema information or extend the XML Schema language with private data. An excellent example is JAXB, the Java/XML binding specification.
JiBX is a framework for binding XML data to Java objects. It lets you work with data from XML documents using your own class structures. The framework handles all the details of converting your data to and from XML based on your instructions, and performs the translation between internal data structures and XML with very high efficiency while still allowing you a high degree of control over the translation process.
rtfx converts RTF files into a generic XML format. It focuses on keeping metadata such as style names rather than every bit of formatting. This makes it handy for converting RTF documents into a custom XML format (using XSL or an additional processing step). It used to be called 'rtfm', but was changed due to a naming conflict.