Here are the feed subscription URLs to add new feeds to some of the most popular feed readers out there:
The Old Reader
%s should be replaced by the feed URL you are going to add. You can use these to add custom readers to extensions like Chrome’s RSS Subscription Extension.
I couldn’t find the URLs for Digg Reader and Blogovin which seem to be popular among many. If your feed reader doesn’t give a good public API it isn’t worth staying at anyway.