There are many useful tools that can significantly improve productive development and deployment of GML-based tools and applications. Here are some of the best!

Parsers

Xerces-J
A general-purpose parser implemented in Java, with preliminary support for the XML Schema specification. Available from the Apache XML Project (open-source).

XSV
Another validating parser, implemented in Python. Available from the Language Technology Group at the University of Edinburgh (open-source).

MSXML 3
A general XML parser from Microsoft. Implements the XSLT and XPath standards.

Editors

XML Spy
An integrated development environment for working with XML documents. By Altova (commercial license, demo available).

XSLT processors

Xalan-Java
This XSLT engine can be used from the command line, in an applet or a servlet, or as a module in some other program. Available from the Apache XML Project (open-source).

SAXON
Another XSLT engine implemented in Java. The download includes documentation, source code, Java executables, and sample code. By Michael Kay.

Graphics

Adobe SVG Viewer
This is a plug-in that your Web browser can use to render graphics in SVG (Scalable Vector Graphics) format. By Adobe (free).