jQuery UI (User Interface) is a comprehensive, open-source library that serves as an extremely useful tool for web developers. With a plethora of powerful features and a user-friendly interface, developers can effortlessly create stunning and interactive websites. Whether you are a seasoned developer or just starting out, jQuery UI provides an extensive range of customizable widgets and interactions to enhance the user experience.

One of the standout features of jQuery UI is the ThemeRoller, a brilliant tool that allows developers to create unique and visually appealing themes for their websites. The ThemeRoller provides an extensive collection of pre-designed themes, or users can customize every aspect of their theme to fit their specific requirements. This level of flexibility truly sets jQuery UI apart from its competitors.

When it comes to compatibility, jQuery UI works seamlessly with all major browsers, ensuring a consistent experience for users regardless of the platform they are accessing the website from. Additionally, developers can benefit from jQuery UI's responsive design, which is crucial in today's mobile-first era.

While jQuery UI has established itself as a popular and reliable choice for developers, it faces competition from other libraries in the market. One such competitor is Bootstrap, developed by Twitter. Like jQuery UI, Bootstrap offers a range of pre-designed components and provides extensive documentation for easy implementation. However, it lacks the customization options that jQuery UI offers, limiting its flexibility for developers with unique design requirements.

Another notable competitor is Foundation, an open-source framework developed by ZURB. While providing a similar range of components to jQuery UI, Foundation places a heavier emphasis on responsive design, making it a suitable choice for developers focused on mobile optimization. However, jQuery UI's vast array of widgets and interactions gives it an advantage in terms of versatility and functionality.

In conclusion, jQuery UI remains the go-to choice for developers seeking a comprehensive library with extensive customization options and cross-browser compatibility. With its extensive range of widgets, interactions, and the powerful ThemeRoller, jQuery UI continues to push boundaries in the web development industry.