FeedWordPress is an Atom/RSS aggregator for WordPress. It syndicates content from feeds that you choose into your WordPress weblog; the content it syndicates appears as a series of special posts in your WordPress posts database. If you syndicate several feeds then you can use WordPress's posts database and templating engine as the back-end of an aggregation ("planet") website. It was developed, originally, because I needed a more flexible replacement for Planet to use at Feminist Blogs.
FeedWordPress is designed with flexibility, ease of use, and ease of configuration in mind. You'll need a working installation of WordPress (version 3.0 or later), and also FTP or SFTP access to your web host.