The lxml XML toolkit for Python
Fast XML parser library in C
X3D is the open-standard format for 3D graphics scenes on the Web.
XMLJ is a Java XML Editor and validator project.
EasyML serialization library, to and from XML, similar to Gson
eXist-db is a feature rich Open Source native XML database
Simulatoralive's Java libraries and programs
NO LONGER MAINTAINED
File validation and characterization
A library to convert between braille and MathML
xslet project provides some XSL libraries and applets.
This program allows embedding books in fb2 format in one, super-book.
A project to define templates for XML markup, and tools to edit them.
In memory database for XML files