XPath Visualizer
Today I found myself writing a bunch of XPath expressions. It was cumbersome to run the full program to test out my expressions so I headed off to live.com in search of a tool. XPath Visualizer was exactly what I needed. You point it to your XML file, type in your query, and click Select Nodes. The result of your command is displayed right in the window. It's quick and effective feedback that has really sped up my development.
