Wikipedia
EXSLT
EXSLT is a community initiative to provide extensions to XSLT, which are broken down into a number of modules, listed below.
The creators (Jeni Tennison, Uche Ogbuji, Jim Fuller, Dave Pawson, et al.) of EXSLT aim to encourage the implementers of XSLT processors to use these extensions, in order to increase the portability of stylesheets.
Downloadable packages that implement the extension functions and templates are provided at the official web site for XSLT processors which don't support a particular extension.