Very useful! Instead of "optically scanning" all the first level comments I can collapse each comment after reading.
NavigComments @ Hacker News
https://chrome.google.com/webstore/detail/aelihpmbbbpmljkdco...
Supports hiding comments marked as read.
Works only with http://, I have to add https:// one day, but I always forget it... Also some other things should be improved, then I could update NavigComments @ LWN too.