The folks at Attensa make a slick RSS aggregator that integrates with Outlook and provides a River of News-style view with priortization based on what feeds you use the most.
Not too long ago, they released version 2.0 of the Attensa for Outlook product. Along with a ton of other great features, it includes synchronization with the Windows RSS Platform, so when you hit that RSS button in IE, the feed can automatically show up in Attensa -- which is great if you're using Attensa as your primary RSS reader.
This has been one of our guiding principles in IE7 -- you don't have to read your feeds in IE. By using the open APIs of the RSS platform, developers of innovative new RSS readers like Attensa can gain access to the orange button in IE7 that Steve Rubel loves.
Attensa works with Outlook 2000, Outlook XP and 2003 (Outlook 2007 support is in the works). If you're an Outlook user, you should definitely check it out. It's free -- so it's an easy choice if you're looking to try out something new.