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.
Apache SpamAssassin is an email filter used to identify spam
Apache SpamAssassin is an extensible email filter which is used to identify spam. Using its rule base, it uses a wide range of advanced heuristic and statistical analysis tests on mail headers and body text to identify "spam", also known as unsolicited bulk email. Once identified, the mail can then be optionally tagged as spam for later filtering. It provides a command line tool to perform filtering, a client-server system to filter large volumes of mail, and Mail::SpamAssassin, a set of Perl modules.
Rave is a widgets-based, web-and-social mashup engine
Apache Rave is a new web and social mashup engine. It will provide an out-of-the-box as well as an extendible lightweight Java platform to host, serve and aggregate (Open)Social Gadgets and services through a highly customizable and Web 2.0 friendly front-end. Rave is targeted as engine for internet and intranet portals and as building block to provide context-aware personalization and collaboration features for multi-site/multi-channel (mobile) oriented and content driven websites and (social) network oriented services and platforms. For the OpenSocial container and services the (Java) Apache Shindig will be integrated. At a later stage further generalization is envisioned to also...
XPath for Java Objects
A Java-based implementation of XPath 1.0 that, in addition to XML processing, can inspect/modify Java object graphs (the library's explicit purpose) and even mixed Java/XML structures.