Online programming editors/debuggers are getting popular. Here are the couple of them which I use from time to time:

• IDEOne – a broad choice of programming languages (there is even BrainF**k, however, there is no XSLT)
• jsFiddle – a nice IDE for JavaScript.
• jsBin – another one for JavaScript
• XSLTCake – useful IDE for XSLT. It is still ‘beta’, but it offers three types of XSLT processing (JavaScript, web service or .NET 4.0)
• XML Playground – another cool IDE for XSL templates. Unfortunately it doesn’t support XSLT 2.0
• Rubular – IDE for checking regular expression written in Ruby
• XSLTTest – Small and neat application for XSLT. Supports XSLT 2.0.