SteemReports - Trending Comments
EDIT: I've just upgraded this report, and made a post about it here.
I've been thinking that it's a shame there's an easy way to see which posts are currently trending, but no equivalent for comments.
After developing something along these lines, I must say, I'm a bit disappointed with the variety and quality I've seen so far, but here is the latest offering anyway. At least it shines a light on a dark corner of the blockchain.