If you are working on iOS, Swift is a great choice. It is elegant, easier to learn (than Obj-C) and powerful.
(Update 9/30/16— This article is two years old. You should check out other stuff)
You need to checkout these open-source projects if you want a headstart in Swift. The Swift Manual is comprehensive, but nothing like diving into code and learning from what is implemented. These are not in any particular order of ranking. Just cool stuff you shouldn’t ignore.
[It is a HTTP networking library. It is easy to use Alamofire for asynchronous network communication, chaining responses for orchestration, etc]
[Surge is based on Accelerate framework, a C-based API for performing matrix math, digital signal processing, and image manipulation. In other words, it is very fast for crunching complex math, speech and image signals]
[Dealing with JSON is a pain if you don’t have support for it. Swifty JSON does exactly that —it deals with the pain so that you don’t have to]
[This is a really cool project(s). It extends Swift with helper functions and extensions]
[Similar to above, this is set of standard extensions for Standard types and classes]
SwiftTask - Promise + progress + pause + cancel, using SwiftState (state machine).
[State machines in Swift. This has lot of use cases for creating business workflow tasks along with others]
HanekeSwift - A lightweight generic cache for iOS written in Swift with extra love for images.
[Looking at the code, still seems early stage. But, lightweight caching systems are a win]
[Auto layout is a system that lets us layout our app’s UI by creating a mathematical description of how elements are related to one another. This project makes it super simple]
[There are several BDD style frameworks for Swift already. I think this is cool because it is pure Swift. There are no dependencies on stuff like NSObject.]
Quick - A behavior-driven development test framework for Swift and Objective-C.
[Another BDD test framework if you are looking for both Obj-C and Swift.]
These are ten projects out of several hundreds that I personally find interesting and fast moving. They are also good projects to dig into the source code and learn Swift fast. Are you ready to build something cool? Go on, do it.