Go to together.Jolla.com, sort by votes...

https://together.jolla.com/questions...s-desc/page:1/

That's pretty much my list though some of those that are listed as implemented are best described as experimental (translation = not working).

CalDAV, CardDAV, SIP are my main three.

But more so than Jolla 'implementing' these I'd rather see them open up the accounts API and settings UI so that other people can do it.