Please add/correct to give an idea of what actually works now.


XBL1, or XBL
============
http://developer.mozilla.org/en/docs/XBL:XBL_1.0_Reference is the
thing that comes closest to what is actually implemented in releases
of Mozilla browsers up to at least Firefox 3.
- http://mozilla.doslash.com/xblschema holds the RNG and generated XSD
- http://spket.com/xul.html shows an editor with XBL specific
functionality (code completion mainly(?)) and
http://addons.mozilla.org/en-US/firefox/addon/3953 is a reference tool
- google for XBL and you'll find working examples (Mozilla extensions,
HTML+XBL and a few SVG+XBL)

sXBL (s=SVG)
============
http://www.w3.org/TR/sXBL/ ('eternal draft, including Schema') is
partly implemented in Batik 1.7 versions, see
http://wiki.apache.org/xmlgraphics-batik/SupportedSVG12Features for
current status

XBL2
====
http://w3.org/TR/xbl (candidate recommendation since March 16) is
being implemented by Mozilla, to replace XBL1 at some point (after
Firefox3 for sure)