If you want to execute arbitrary fetchXML code, dont forget about the bids reporting extension.
While developing some custom code best suited to a fetchXML query, I wanted a rapid way to execute fetchXML for a complex query. A small test app would work, and there is even a simple codeplex project for exactly that here http://crm2011fetchexecute.codeplex.com/. However I recalled executing fetxhXML in the CRM 2011 bids reporting extension so if you have that installed simply open one of your CRM based reports and make a new dataset and use the query designer there to help debug those complex fetxhXML statements!
