Here are the feed subscription URLs to add new feeds to some of the most popular feed readers out there:
Newsblur
http://www.newsblur.com/?url=%s
Feedly
http://cloud.feedly.com/#subscription%2Ffeed%2F%s
The Old Reader
http://theoldreader.com/feeds/subscribe?url=%s
Bloglines
http://www.bloglines.com/login?r=/sub/%s
The %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. 😉