XPath is one language, if call it so which is increasingly being used everywhere. Is used from development to testing environment. In the testing fields, its been used as a add on for many testing tools. Consider SoapUI which is used for testing and developing web services. Over here XPath can be used to trace a XML and find out specific words.

In selenium RC, we could use XPath for in java programs to search a image, button text field to find the data we need. Many times it isnt shrwed to write our own XPath scripts, it takes quite a long time. One of the editors I found online is at http://slesinsky.org/brian/code/xpath_checker.html.

This easy to use editor is a add on for mozilla. You can just RIGHT click on a table for example in the HTML to get the XPath query for that.Some of the screen shots are as below :

Examples of XPAth queries are :
selenium.click(//input[@type=image]);
selenium.click(//input[@value=Submit]);

These examples are with selenium testing tool