4xt.org
Collaborative resource for XSLT XT users as well as a XT showcase. Discussions list, news articles, documentations, FAQ, downloads and links to other XT resources.

XSLTester
A free open source visual XSL development environment. First created in 1998, this is a great tool.

XSL Startkabel
Portal site for XSL and XSLT only.

EXSLT
EXSLT is a community initiative to provide extensions to XSLT. They are trying to encourage the implementers of XSLT processors to use these extensions, so that your stylesheets can be more portable.

Examplotron
The purpose of examplotron is to use instance documents as a lightweight schema language-- eventually adding the information needed to guide a validator in the sample documents.

Hysen, Gordon - XSLT
Resources, downloads, links, and xsl-list.