A MacRuby developer is responsible for creating and managing applications specifically for the Mac OS X operating system. They utilize MacRuby, a version of the Ruby programming language that is tightly integrated with Apple's Cocoa libraries. Their tasks include designing, coding, testing, debugging, and maintaining applications. They work closely with UI/UX designers to create user-friendly interfaces, ensure the compatibility of new software with existing systems, and resolve any issues that arise. Additionally, they may also be involved in providing technical support and updates for applications post-deployment. Their goal is to create efficient, high-quality software that enhances user experience on Mac systems.