I'm sure we all feel that the world needs another RSS Aggregator, and lucky for us... someone has written one :)
Darshan Singh has created an excellent RSS aggregator using Windows Forms (and C#). RSSConnect supports skinning of the post viewing pane, searching and indexing of posts using the integrated database support as well as a number of other customization points.
Download RSSConnect from PerfectXML.com
Seriously though, it looks like it has some interesting features around searching/indexing, so I'm sure it will add to that great pool of desirable aggregator features that all aggregator writers are helping to advance.