There have been quite a lot of significant announcements during today's WWDC, but I think my personal favorite has got to be Slide Over for iOS 9.
Slide Over allows users to "slide over" a second bar long the side of the screen that will contain a second app - in other words you can have two apps up and running at the same time. This, combined with Split View, will let you browse in Safari while checking your messages, hop back and forth between email and your notes without exiting anything, and more. It seems like such a small thing, but being able to use two apps at the same time seems like it'll quickly become one of those "How did I ever live without this?" kind of things.
But what do you all think? Is this going to be a handy new feature or does it seem kind of useless?