Sitecore XPath Builder
One of the defining characteristics of a Content Management System (in my humble opinion) is the ability to actually manage large amounts of content. In the case of Sitecore, there are some underlying architectural niceties which make this task far easier. The biggest one is the fact that there is a hierarchical tree structure, which is exposed via an API which easily allows for relationship-based querying. One of the ways this hierarchy is exposed is via XPATH. Because XPATH is an extremely powerful language for querying, it’s also very complex to ensure you have built your queries correctly, so Sitecore provided a tool to help with this task.
These queries can be used in various places, both in your front-end code for displaying navigation or related items, but also within many aspects of the Sitecore user interface itself where you can filter items when making references. In early versions of Sitecore, this was easy to find - critical since XSLT was one of the primary ways to create front-end components. As XSLT waned in use, this interface was buried in later versions of the product.