Atlassian Developer Toolbox Plugin
The Atlassian Developer Toolbox is a set of tools used to improve developer productivity. It includes the REST API Browser which allows a developer to browse, discover, and test Atlassian's rich REST and JSON-RPC APIs.
After you install the Developer Toolbox, go into the product's Administration page. You'll find the "Developer Toolbox" category on that page.
For more information, visit the Marketplace listing.