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IBM XSL Editor

IBM has been a forerunner in providing great tools, SDKs, and support for the XML community. Just one of the many XML software tools it provides is an XSL/XML editor. IBM's XSL Editor features include the ability to create, import, and edit XSL and XML documents. It allows you to set "break points" on the style sheet and source document. It also features a nifty collapsible tree view. Check it out.

IBM XSL Editor