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.
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.
Maven is a project development management and comprehension tool.
Maven is a project development management and comprehension tool. Based on the concept of a project object model: builds, dependency management, documentation creation, site publication, and distribution publication are all controlled from the declarative file. Maven can be extended by plugins to utilise a number of other development tools for reporting or the build process.
A library for developing geospatial applications.
Apache SIS provides data structures for geographic data and associated metadata along with methods to manipulate those data structures. The library is an implementation of GeoAPI interfaces and can be used for desktop or server applications.