Here are some non pet store but a real world , non trivial , and open source application that use Spring Boot 2.

1. Thingsboard which is an IoT platform with the microservice architecture. Built with Spring Boot , Spring Security , Spring Data , Spring MVC etc.

2. Flowable is a business process engines that are based on Spring and have already upgrade to support Spring Boot 2.0

3. Spring Initializr is the backend web API that can quickly generate a sample spring-boot project. It is exactly the backend API that powered the famous start.spring.io.

4. Kafdrop is the web client that managing Kafka. Built with Spring Boot, Spring MVC, Freemarker etc.

5. Klaw is another web client that managing Kafka. Built with Spring Boot, Spring MVC, Spring Security,Spring Data JPA and Thymeleaf etc.

6. The backend of the Corona-Warn-App which is an app that helps trace infection chains of COVID-19 in Germany.Built with Spring Boot, Spring MVC, Spring Security,Spring Data JPA, Bean Validation etc.

7. CloudFoundry User Account and Authentication (UAA) Server is a multi tenant identity management service used in Cloud Foundry , but also available as a stand alone OAuth2 server. Built with Spring Boot , Spring MVC , Spring Security , Spring JDBC etc.

8. Apollo is the distributed configuration management system to allow you change your application 's configuration to take effect in the real time without starting your application. Can be used to implement grayscale release too. Built with Spring Boot , Spring Cloud (e.g Eureka) , Spring Session , Spring Security , Spring MVC , Spring Data JPA etc.

I know for example this two:

• Project Sagan: https://github.com/RameshMF/sagan
• Zipkin: https://zipkin.io/

More can be found under: https://www.javaguides.net/2018/10/free-open-source-projects-using-spring-boot.html