XQDebug is a browser-based debugger for MarkLogic XQuery applications. It is useful for debugging native XQuery applications, RESTful web services and XQuery code in general. Once you have installed XQDebug on a MarkLogic host, you can connect to any of that host's MarkLogic application servers which will stop all requests to that appserver for debugging. Then from the main XQDebug window you can select the stopped request to debug, view the current expression and variables, view the call stack, set breakpoints, create watch expressions and step through your XQuery code.
XQDebug was created by Keith Breinholt. It has been tested on MarkLogic 4.1-5+ and 5.0-1 from FireFox 3.5+ and IE 8.0. You can see a presentation that Keith gave at the MarkLogic 2010 User Conference here.