Urchin is a Web based RSS aggregator, implemented in Perl. There's plenty half baked takes on the same idea, but Urchin seems to have a nice clean logical design from top to bottom. Someone actually put some thought into how to build the backend of an RSS aggregator. In particular, it's DB schema was designed to cleanly represent feeds as RDF triples, and support the generation of RSS by querying the RDF store. Base modules have been speced to support ingesting any number of formats, while XSLT is used to generate output feeds.
Ben Hammersley gave Urchin a fairly positive review, although I would have liked to have heard if he actually put Urchin to use for an extended period of time.
At best Urchin could be the start of the aggregator platform I dream of. An extensible UI would have to be grafted onto it. At worst, folks could look at the DB schema for guidance on storage backends for RSS.