See also Distributing lists of feeds and Slash Style Sites.
Lists of RSS sources
NewsIsFree A large site that collects RSS and publishes feeds it finds.
My.UserLand New Channels
My.UserLand Channels that have been updated in the last 7 days
Newsfeeds A blog that specializes in promoting new feeds. Linked to the "Headline Viewer" RSS Reader.
Meerkat An aggregator. Browse around 'til you find what you like, and add &_fl=rss&t=ALL to the URL. Instructions here.
Moreover A typical moreover feed looks like this.
The Amphetadesk list of known sources in XML format.
XMltree. A directory of XML sources (not just RSS)
The Headline Viewer core list
10.am. Similar to Moreover.
Network 54 Browse to a page on the site and add ;xml=rss to the end.
Syndic8 An RSS evangelism project with lots of feeds.
Most popular channels
Our Favorite Songs This page lists the most recently changed RSS files managed by the cloud that connects all Radio users together.
Google search for PHP-Nuke style Portals
Another Google search for PHP-Nuke style Portals
Unusual Sources and queries
Any Yahoogroups mailing list. Create a URL like http://groups.yahoo.com/group/the_group/messages?rss=1 where "the_group" is the name of the mailing list.
Moreover Query. Create a URL like http://p.moreover.com/cgi-local/page?o=rss&query=p2p where the query parameter is a properly escaped search string.
Major RSS Generators
Here's the likely location of the RSS file. Add it onto the main URL and see if it works.
Manila based blogs - /rss/rss.xml
Slashcode based sites - /articles.rdf
Scoop based sites - /backend.rdf
php-nuke based sites - /backend.php