Octora is a beta RSS search engine.  You can input a keyword (tag) and the search engine will find all relevant RSS feeds for the keyword.
Try Octora at octora.com.

We all realize paying close attention to our site’s keyword density is important.  Site owners need to make sure their topic’s most often used keywords are related to the items people search for repeatedly.  Workboxers, a 9rules blog, has realized it can be more lucrative, and easier, to focus on driving up the keyword density [...]