If you follow some of the more popular YouTubers, you might have noticed some spiffy new screens at the end of their videos as of late. Those are video End Screens, and now they're available to everyone. End Screens provide a handy interface for linking to other videos and sites inside your video without all the clunky workarounds people were using before.
End Screens can be integrated into the last 5-20 seconds of your video via the YouTube editing tools. Simply choose End Screen and create the element with up to four items. At least one of them has to be a video, but you can also include links to your website, an approved merchandise/crowdfunding site, a playlist, another channel, or just a subscribe link for your channel.
YouTube says it has been working with some YouTube partners to test the feature recently, which is why you may have seen it pop up. A lot of video creators were using annotations to do the same basic thing before, but that's a desktop-only feature and it looks bad. End Screens will work almost everywhere including desktop (non-Flash), iOS (minimum YouTube v11.03 for iOS9 and v11.09 for iOS8), Android (YouTube v11.04 or higher), and mobile web. It won't work in YouTube Music, YouTube Kids, or YouTube TV right now.