Paperboy has many features making it an excellent choice for aggregating XML data:
Any XML format can be converted to any text based format (e.g. HTML, plain text, or even a different kind of XML).
It is very flexible, and can be applied to a range of application areas (from preparing files for web viewing, to dumping parsed contents of an XML file to the screen, to being the back-end for web-based data systems).
Special XSLT processing functions are added for more complex operations with XML feeds (such as seeing if an item is new, and being able to sort by publishing time).
It allows you to extend XSLT processing capabilities even further by adding your own processing functions.
Paperboy includes secondary programs which work in conjunction with paperboy to add more functionality.
It works on multiple platforms.
It is easy to learn.
It is standards compliant.
It is free/open-source software.