Gregarius is a web-based RSS/RDF/ATOM feed aggregator, designed to run on your web server, allowing you to access your news sources from wherever you want.
WP-AutoBlog is a plugin for WordPress that inserts posts based on RSS feeds. It has been used to generate a single WP site containing all the posts of a WPMU install to provide cross-site search / tagging / categories, etc. It could also power a planet.
FeedWordPress is a plugin that inserts posts into a WordPress site from other RSS feeds. It can be used to create a planet (rss aggregator) on WordPress.