My favorite little-known HN feature (not even mentioned on this link!) is the noprocrast mode. You can set in your profile to be locked out of HN for a set amount of time, after another set amount of time on HN. Prevents procrastination :)
Oh wow, that's a good one! My favorite is being able to change the nav color (`topcolor`). I changed it to the purple brand color from our startup [1].
I can imagine that being used as a security feature for UIs, against phishing attacks: now that you are registered and logged in, choose one of these colours/pictures for this certain, easily visible area of your dashboard. If the use saw a different colour, would know that's phishing.
