Protege is a free, open source ontology editor and knowledge-base framework. The Protege-OWL editor is an extension of Protege that supports the Web Ontology Language (OWL). The release 4.1 or later is recommended.
RDF Validator is a Web Service provided by W3C. Use it to check the validity of RDF/XML documents with respect to the official RDF specification.
XML Schema 1.1 Tools: free and commercial. Online Validator for XML Schema provided by W3C.
The not so Short Introduction to LaTeX2e in your native language: written by Tobi Oetiker. From this document you will learn all you need to know about LaTeX. It will enable you to write your documents with this great tool. You will even learn what LaTeX is, in case you don't have a clue.
The root of the Comprehensive TeX Archive Network directory tree.
The TeX Users Group (TUG) Web site.