Final answer:
Developers use the ServiceNow Platform, specifically the ServiceNow Studio, to create custom applications within ServiceNow. It also includes the ServiceNow Application Creator for more straightforward app development.
Step-by-step explanation:
The ServiceNow tool used by developers to create custom applications is called the ServiceNow Platform. It offers a suite of tools to manage enterprise operations and provides an extensible framework for developing custom applications. One of its central features is the ServiceNow Studio, an integrated development environment (IDE) designed to be used within the ServiceNow platform ecosystem. Developers can build, test, and deploy applications directly within this tool. For more in-depth customizations or applications, developers have access to the ServiceNow Application Creator, which streamlines the process of creating scoped applications without requiring expert knowledge of coding.