Here are some of the popular tools for iOS app development:

1. Xcode: Xcode is a powerful integrated development environment (IDE) from Apple that provides everything developers need to create high-quality iOS, iPadOS, macOS, watchOS, and tvOS applications.
2. Swift: Swift is a powerful, open-source programming language developed by Apple for iOS, iPadOS, macOS, watchOS, and tvOS development.
3. Objective-C: Objective-C is an object-oriented programming language used for iOS and macOS development.
4. Cocoa Touch: Cocoa Touch is a framework for building software applications on iOS. It provides a collection of libraries and tools that are essential for developing iOS applications.
5. Interface Builder: Interface Builder is a visual design tool that makes it easy to create and prototype user interfaces for iOS applications.
6. Core Data: Core Data is a framework for managing an object graph of data, providing a rich set of data persistence and management options.
7. Instruments: Instruments is a performance and debugging tool that provides developers with the ability to profile and analyze the performance of their iOS applications.
8. TestFlight: TestFlight is a popular platform for testing iOS apps before releasing them to the App Store.

These are just some of the many tools available to iOS app developers. The best tool for a particular project depends on the requirements of the project and the skills of the developer.