Sun's XML Pipeline Note at the W3C - language specification

Norm Walsh's sxpipe project at Java.net - open source - language and implementation

My smallx project at Java.net - open source - language and implementation

Apache's Cocoon - open source - chaining of SAX filters in the sitemap.

Markup Technology - commercial - language and implentation - also implements (1).

Orbeon - open source/commercial - language and implentation embedded in product.