One of the most common requirements we have these days is to display RSS feeds on WSS/MOSS sites inside of a web part. Most users want the ability to see just a list of article links, sorted by date, from multiple feed sources - a collection of technology blogs or all the official SharePoint blogs, for example.
There are several free utilities for performing RSS aggregation in SharePoint - inlcuding those that come out of the box with 2007 - but none of them work the way that I needed them to (in fact, I couldn't get the included web parts to work at all in WSS). So I put together a basic RSS Aggregation web part that fits the bill.
The functionality is really quite simple - supply the web part with a semi-colon delimited list of feed URL's, set a few parameters (like number of listings per feed, total number of listings, alternate stylesheet, etc.) and drop it onto a page. The web part will display a list of links from each feed source sorted by publish date in descending order. The formatting is controlled by a set of base and alternate styles that can be included in custom stylesheet or integrated with your base site definition styles.
You can download the RssAggregator web part here: http://www.binarywave.com/products/downloads/Free%20Utilities/BinaryWave.RssAggregator.zip. The zip file contains the Visual Studio 2005 solution with full source code (written in C#) and a readme file with setup and configuration instructions. Post any issues or questions as comments to http://www.binarywave.com/blogs/eshupps/Lists/Posts/Post.aspx?ID=3.
Note: I discovered while putting this together that there are quite a few differing feed formats depending on which source you are trying to access. Most RSS 2.0 and ATOM feeds should work but you may need to tweak the XML parsing methods in the source code if you are referencing a feed that uses non-standard or custom formatting in the source XML. If you do make modifications, please post them so everyone can benefit from your changes.