The ASF is made up of more than 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.
OSGi Enterprise Programming Model
The Aries project is delivering a set of pluggable Java components enabling an enterprise OSGi application programming model. This includes implementations and extensions of application-focused specifications defined by the OSGi Alliance Enterprise Expert Group (EEG) and an assembly format for multi-bundle applications, for deployment to a variety of OSGi based runtimes.
Rule based XML->Java object mapping module
The Digester package lets you configure an XML->Java object mapping module which triggers certain actions called rules whenever a particular pattern of nested XML elements is recognized.
Apache Tentacles simplifies the job of reviewing repository releases consisting of large numbers of artifacts.
Apache Tentacles helps the reviewer by automating interactions with the repository containing the artifacts comprising the release.