Amaya

Python

ISO Schematron

Developer Uche Ogbuji has an article A Hands-On Introduction to Schematron on IBM Developer Works

UBL interim implementation

XSLT

ISO Schematron profile

Ken Holman's modified version of the Schematron 1.5 implementation, awaiting the updated version of ASCC Schematron skeleton.

Topologi Professional Edition

Java

Schematron 1.5

Valid XML and SGML with Schematron. Validate from markup editor, tree editor or on multiple files. Generate Schematron "usage" schema by sampling document set. Pretty-print Schematron schemas.

Topologi Markup Editor

Java

Schematron 1.5

Topologi Proxy Validator

Java

Schematron 1.5

Topologi Schematron Validator

MicroSoft Windows/MSXML

Schematron 1.5

(Note: Email from MicroSoft reports that they have used Schematron engines as part of their regression tests for MSXML.) Developer Eddie Robertsson has article aboutEmbedding Schematron in other schema languages and anIntroduction to Schematron at XML.COM

Schematron.NET

MicroSoft .NET

CD draft ISO Schematron

Native implementation. Developer Daniel Cazzulino will be presenting a paper All about Schematron at a developer's conference in October 2004.

Scimitar

Python

CD draft ISO Schematron

Developer Uche Ogbuji has an article A Hands-On Introduction to Schematron on IBM Developer Works

James Clark'sJing

Schematron 1.5

Java/XSLT (SAXON)

Expected to be updated to draft ISO Schematron

XMLForm Schematron

C++

Schematron 1.5?

Part of Apache Cocoon. See alsoValidation Concepts for site validation.

4Suite Schematron

Python

Schematron 1.5?

Use Scimitar instead (above)?

Sun MSV Schematron add-on

Java

Schematron 1.3?

Semi-experimental add-on to Sun's Multi Schema Validator

xml::schematron

Perl

Schematron 1.5?

Developer Kip Hampton has article at XML.COM.

Ewins' Jaxen

Java/Jaxen

CD draft ISO Schematron

Experimental rewrite of Jaxen to support Schematron better. Native implementation.

Academia Sinica Schematron 1.5

XSLT (Most implementations of XSLT: problem reported with Xalan)

Schematron 1.5

Implementation modularized into a skeleton and "meta-stylesheets" that allow various output formats and customization. Includes Screamathon torture test, for trialling different implementations of XSLT, which has been incorporated into the draft International Standard as an Annex. Code by Oliver Becker, Rick Jelliffe and others. Schematron-message can be used in conjunction with any XSLT implementation that outputs vi-style error messages and any text editor that reads vi-style error messages (vi, emacs, XED, etc) to create a simple validating editor. 
This is an open source "seeding" implementation: implementations are known to be been written derived from this and called from UNIX and Windows command shells, from Visual Basic, C++, and Java. 
Used by, for example, the NISTSchematron Editor Tool 
Developer Rick Jelliffe has aninterview at XMLHACK.COM

PHP Schematron

PHP

Schematron 1.5

Academia Sinica Schematron 1.3

XSLT (James Clark's XT and others)

Schematron 1.3

Original reference implementation by Rick Jelliffe. Now obsolete.

ZVON Schematron

XSLT 1.0

Schematron 1.0

See XML Journal for an article on integrating ZVON Schematron into XML Spy.