Any site which iframes in another site where you're expected to be logged in. For example, Disqus comments, Facebook like buttons, Youtube embeds (which work, but show ads even if you've paid for no ads).

There are also lots of one-off cases where developers split functionality across multiple domains in ways that were fine at the time but now get blocked. These are all fixable, unlike the iframing case, but it's still a lot of sites.

I think it's more nuanced than just blocking ALL third-party cookies. If you have visited the site before (e.g. facebook.com), my understanding is that those cookies are allowed. It's only blocking third-party cookies from sites you have never explicitly visited before.

Atlassian's HipChat web app breaks (in Gnome Web, which also disables third-party cookies).

<snark>So, nothing you'd want to use.</snark>