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.
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.
Solr is a full-text search server
Solr is an open source enterprise search server based on the Lucene Java search library, with XML/HTTP and JSON, Ruby, and Python APIs, hit highlighting, faceted search, caching, replication, and a web administration interface.
Gump provides large scale continuous integration for various open source projects.