The ASF is made up of over 100 top level projects that 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.
Software components for semantic content management
Apache Stanbol is a modular software stack and reusable set of components for semantic content management.
Torque is an object-relational mapper for Java.
Torque is an object-relational mapper for Java. In other words, Torque lets you access and manipulate data in a relational database using java objects. Unlike most other object-relational mappers, Torque does not use reflection to access user-provided classes, but it generates the necessary classes (including the Data Objects) from an XML schema describing the database layout (which can either be written by hand or generated from an existing database). The XML schema can also be used to generate and execute a SQL script which creates all the tables in the database.
Validation Framework and Routines
Commons Validator provides the building blocks for both client side validation and server side data validation. It may be used standalone or with a framework like Struts.