RSS-Extensions Wiki is like Microformats documentation for RSS. It is a central point for looking up the various encodings, formats, uses, and resources for extensions like creativeCommons, Yahoo Wather, or Apple iTunes RSS.
There is a link to some good articles on how to extend RSS by adding your own namespaces. A discussion like this is currently ongoing in GeoRSS on how best to add named locations, addresses, non-Political features (oceans for example), and general areas (like USGS regions). It's a mixture of what is understandable by the user (web devs & programmers who will actually use these extensions), covers most use cases (so users are happy and use the extension and don't go and try to make their own), and hopefully builds or uses existing formats.
Pat Cappelaere, who developed the very cool GeoBliki, proposed the following extension:
SÃtio do Mato
which does a good job of providing a fairly simple format for users to produce and parse when developing tools. This is important because while people aren't really meant to read RSS, they're still developing the tools to handle the RSS, and therefore need to understand it.
RSS-Extensions Wiki also a great list of RSS Syndication Hacks, which is full of ideas for projects, or integration into your existing projects, like programming/code todo's (@todo in source or repository commits), or finding media to add to your iTunes.