Ask HN: What HN related third-party app, extensions, etc. do you use?

One tool per limit each top-level comment please — to make it easier to get a sense of what tools are popular by votes.

If possible, provide name, TLDR, URL, and links to any exiting notable HN threads, etc.

Examples include: hnreplies.com, hn.algolia.com, stylometry.net, etc.