FeedWordPress is an Atom/RSS aggregator that syndicates content from feeds that you choose into WP; 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.
Have I mentioned BlogLines.com? It really is a cool service, and what’s more – it’s free.
BlogLines is an RSS aggregator; don’t let that big name scare you – basically it allows you to get content from a bunch of sites, organised into your own directories and catgories, all in one place. It’s perfect for keeping up to date on news, reading your daily blogs, or perhaps receiving certain types of updates and things like that.
There’s not much I can say, other than it’s a very well-made system, it’s entirely web-based (so you can access it from all machines you use) and it has some great features which make managing your feed subscriptions really easy. Check it out and see what I’m talking about.