This doesn't have to be as simple as chaining a few XSLT transformations together, as Plagger filters can carry out additional actions (such as e-mailing the results to you, calling on the power of Perl modules to create summaries, and so on).
As a learning exercise, I've built Planet Subversion (edit I've updated the URL to point to the official domain). This takes feeds from a number of different sources and builds a "Planet" site from them. And, of course, with Plagger being open source, it's easy to contribute any fixes back to the author.
Please let me know if you use Subversion and can recommend any other feeds to add.
If you'd like to produce your own aggregation site using Plagger, here's the config file that I'm using for Planet Subversion.
- module: Subscription::Config
- module: Bundle::Planet
duration: 7 days
title: Planet Subversion
description: Everything Subversion
dir: [set this to your preferred output directory]