On: Ask HN: Better to have optional signups or force signups/logins?

It depends on what you build. Generally you should require signin for things that need signin - requiring them for things that don't will drive users away. You can still uniquely identify users if you need to, and then convert them to an actual account if/when they signup.

That's pretty much what I was thinking. The discovery part of the app would be open but the profile based things would of course need a login.

What ways can you uniquely identify users who aren't logged in?

browsers are surprisingly unique to each user; see "browser fingerprinting" https://panopticlick.eff.org/

You could also just use cookies, which is perhaps a more socially conscious way of handing it.

Browser fingerprinting fails for cases where a computer is shared. That includes both home and public terminals. Not to mention it creeps people out and lacks any type of security.