Many of you surely remember Technorati. It was one of the first popular RSS blog aggregators. was it a directory too? I forgot.

https://web.archive.org/web/20080529000125/http://technorati...

Nowadays, I find Feedly topics a good place to explore RSS sources. I believe it is human curation, so it is kind of a directory too (though less "indie" and not restricted to blogs). You can sort by Followers (=popularity) and articles/week.