I have been submitting a few patches for Drupal's aggregator module recently and working closely together with Dries (Drupal's Benevolent Dictator for Life) to create the planet. He mercilessly reviewed (and applied!) my patches, commented on my ideas and suggested lots of improvements. Without him the planet wouldn't be here today.
The now improved aggregator module is capable to create a Planet site like Planet Drupal in a few minutes and mostly out-of-the-box, requiring no changes to the code. There are a few minor issues which we need to sort out, but the Planet is there now, and you can subscribe to it in any RSS feed reader.