I'm actually writing a chrome extension right now for hn just for the sake of learning. I do believe there is a plugin that does this already (for chrome - search hacker news in the store) - but I would be happy to add it to mine. Right now, I just added a link to the right of all the user names that says "profile for xxx" and if you click it, it pulls their profile details into the page.

I will add this because I needed an excuse to use the html5 local data store. Also, I'll post the code to github when it's done and announce here.

Anything else anyone wants to see from an extension like this?

This can be done really easily in userscripts (accepted in Chrome too).

  content-area a:visited {display:none;}

Actual coding left as an exercise to the reader.

Will that automatically takes the top 15 if 5 links are already visited?

No it won't, but its just a quick hack.

that's actually a great idea..