Storehouse is still one of the most beautiful apps on the App Store when it comes to animations and this week they have open sourced their animation engine. Built on top of CADisplayLink, they recommend this mainly if you're looking to do physics based animations that interact with gestures.
Great little library from Christian Menschel which lets you easily keep track of your app's frame rate while in development. It colours the status bar and gives you an FPS counter so you can always be aware of when you're dropping below 60 FPS.
Assertions, preconditions, preconditionFailures and fatalErrors. Swift brought with it much finer grained control of how you handle your assumptions. Even if you think you know everything about them though, Mike Ash digs deeper than you probably will have done so you should still give it a read.
Olivier Halligon explains a whole load of useful techniques for using lazy initialisations in your Swift projects.