In short, XMUltra is the Ant of feed processing.
XMUltra a Java and XML based feed processing framework. It has a variety of utilities available for feed processing. These include tools for:
This is really only the tip of the iceberg. XMUltra can take over any housekeeping task where you used to use shell scripts, batch files, or cron jobs. It is easy to extend, so you can create a custom processor to fit your needs.
XMUltra was originally created by Knight Ridder Digital. Later, when the technology was inherited by McClatchy Interactive, they decided to open-source it so that development could continue.
XMUltra was originally designed for news feeds, and it features a separate plugin architecture specifically for this. For a more detailed history, you can read this blog post.
The examples zip file contains increasingly complex examples of how you can use XMUltra (as well as the libraries and DTDs you need). For more information, check out the documentation available in Subversion.
If you are interested in contributing, check out the project page.