I got several questions what's behind siblogs.com, especially about the feed parsing engine.
No, I didn't write my own classes for parsing RSS and Atom feeds. I use slightly modified versions of open source libraries RSS.NET and Atom.NET. Both projects seem quite but are suitable for use in your application. Documentation is not very helpful, but there are some examples that should be enough to figure everything out.
The main trouble with both libraries is the fact that they use .NET XML classes and fail if the XML if not well formed or if there is something wrong with the encoding. In current form it's impossible to make them more forgiving. Fortunately it doesn't seem to be a big problem for si.blogs. I bug authors to make their feeds valid from time to time and they are very cooperative.