Among all the other wacky but useful features in HEY, there's read markers (or at least that's what I call them).

This is how they look:

If you've used HEY email, you'd know that Feed and Papertrail pages can get hard to track because of the sheer volume of emails flowing in.

With a neat marker like in the screenshot above, it's easy to remember how much you've read already and where you need to pick up from.

Good software focuses on tiny improvements. HEY is a great example.