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.
Apache ServiceMix is an open source ESB implementing JBI.
Apache ServiceMix is an open source distributed Enterprise Service Bus (ESB) and SOA toolkit built from the ground up on the semantics and APIs of the Java Business Integration (JBI) specification JSR 208 and released under the Apache license.
Commons JEXL Expression Language Engine
Jexl is an implementation of the JSTL Expression Language with extensions.
Pure Java application for load and functional testing
Apache JMeter may be used to test performance both on static and dynamic resources (files, Servlets, Perl scripts, Java Objects, Data Bases and Queries, FTP Servers and more). It can be used to simulate a heavy load on a server, network or object to test its strength or to analyze overall performance under different load types. You can use it to make a graphical analysis of performance or to test your server/script/object behavior under heavy concurrent load.