The catalogue is an extensive database of many, most, and one day, maybe, all, packages available for TeX/LaTeX. It started out as an example of using BibTeX, but is now fully XML and XSL based. The Catalogue is best browsed online, but refer to xml-catalogue for a nice exmaple of using xmltex. Oh, by the way, you are currently looking at the TeX Catalogue, if you hadn't noticed. The author is Graham Williams.