For email, IMAP has existed for ages. It allows you to see multiple email inboxes in one place (among other things). This is great.
But there is an even stronger need for this functionality with calendars. I want to be able to see my work calendar, personal calendar, and those of certain friends/family, all in one view. Basically the ability to synchronise multiple online accounts/databases with one application. Just like IMAP.
Why doesn’t it exist?
I’ve heard of caldav. But either it doesn’t work, it is not meant to do that, nobody supports it properly, or else I just couldn’t figure it out.
A working version of this is a big thing the world needs.