eXtensible Style Language (XSL) 
W3C's page detailing XSL news, resources, guides, tools, and specifications.

XSL Resources 
Contains links to introductions and overviews, recent specifications, information about the W3C XSL Working Group, articles and papers, software support, style sheets, background information, discussion lists, conferences, and other related sites.

A Proposal for XSL
A proposal for XSL (Extensible Style Language) that is the deliverable for Phase III of the SGML, XML, and Structured Document Interchange Activity of the W3C.

Extensible Stylesheet Language (XSL)
First working draft for XSL from the W3C.

Using XSL and CSS Together
Discusses how XSL can be used as a bridge between complex XML-based documents and the CSS formatting model.

WDVL XSL Page
WDVL's XSL page contains articles and resource for all areas of XSL. Even provides a link to the W3C's XSL Working Draft page.