Ever wonder what Planet Ubuntu runs on? It happens to be a whole CMS. This can be obtained at planetplanet.org.
This doesn't fit so well into sites that aren't meant to focus around feed aggregation however. The Ubuntu Drupal team has come up with a module to fix this.
Here comes the Drupal Planet module. This allows your users to add their own feeds to your own planet. Rather than needing to create a whole new site, the feeds will be listed under /planet of your site. A user can add as many feeds as they want. The feeds get updated during a cron.
Want examples? http://sd.ubuntu-us.org/planet and http://www.ubuntu-qc.org/drupal/planet
The module Drupal Planet can be downloaded at https://launchpad.net/drupal-planet/6.x/1.0.
This module was originally developed by Daryl Houston and code finished by David Giard.
Once again, a great release from the Ubuntu Drupal team. As always, if you have any suggestions for existing modules or ones that would be useful, feel free to fire an email to ubuntu-drupal AT lists DOT launchpad DOT net. Feel free to join the team as well to become active in our development process or to just know what's going on.