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.
Commons JEXL Expression Language Engine
Jexl is an implementation of the JSTL Expression Language with extensions.
Apache River software provides a standards-compilani JINI service.
Apache River software provides a JINI service, which is a service oriented architecture that defines a programming model which both exploits and extends Java technology to enable the construction of secure, distributed systems consisting of federations of services and clients. Jini technology can be used to build adaptive network systems that are scalable, evolvable and flexible as typically required in dynamic computing environments.
Web development framework: separation of concerns, component-based.
Apache Cocoon is a web development framework built around the concepts of separation of concerns (making sure people can interact and collaborate on a project, without stepping on each other toes) and component-based web development. Cocoon implements these concepts around the notion of "component pipelines", each component on the pipeline specializing on a particular operation. This makes it possible to use a "building block" approach for web solutions, hooking together components into pipelines without any required programming.