[mhartl]

This is neat. I've been using this technique ever since stumbling across it in "Building Communities with Software" by Joel Spolsky (http://www.joelonsoftware.com/articles/BuildingCommunitieswi...). Money quote:

Q. Why don't you have some kind of system so I can see what posts I've already read?

A. We have the best system that can be implemented in a distributed, scalable fashion: we let everyone's browser keep track of it. Web browsers will change the color of the links you've already visited from blue to purple. So all we have to do is subtly change the URL for each topic to include the number of replies available; that way when there are additional replies the post will appear in the "unread" color again.

Anything more elaborate than this would be harder to build and would needlessly complicate the UI.