The ASF is made up of nearly 150 top level projects which cover a wide range of technologies. Chances are if you are looking for a rewarding experience in Open Source, you are going to find it here.
Apache Isis is a framework for rapidly developing domain-driven apps in Java
Apache Isis is a framework for rapidly developing domain-driven apps in Java. Write your business logic in entities, domain services and repositories, and the framework dynamically (at runtime) generates a representation of that domain model as a webapp or as a RESTful API. For prototyping or production.
Apache OFBiz (The Apache Open For Business Project) is an open source enterprise automation software project
Apache OFBiz (The Apache Open For Business Project) is an open source enterprise automation software project. By open source enterprise automation we mean: Open Source ERP, Open Source CRM, Open Source E-Business / E-Commerce, Open Source SCM, Open Source MRP, Open Source CMMS/EAM, and so on. It is one of the few apps of this type to be developed by a community, rather than one corporation.
Bytecode Engineering Library
The Byte Code Engineering Library is intended to give users a convenient possibility to analyze, create, and manipulate (binary) Java class files (those ending with .class). Classes are represented by objects which contain all the symbolic information of the given class: methods, fields and byte code instructions, in particular.