A list of awesome 3rd party libraries and tools for Apple platform development, iOS and Mac. This is not an exhaustive list, just an opinionated set of resources. I highly recommend you check Github's Objective-C and Swift Trending Page often.

Libraries & Frameworks

iOS UI
DZNEmptyDataSet Show an special image when no data is available
JVFloatLabeledTextField Give context to iOS users' text fields
ClusterPrePermissions Ask for iOS permissions twice
Tweaks Tweak parameters at runtime in app on iOS
M13ProgressSuite A wide range of progress indicators for iOS
SpinKit Several indeterminate loading spinners
MBProgressHUD For overlaying progress on top of a view controller
TTTAttributedLabel Better attributed strings in UILabels
AsyncDisplayKit Asynchronous user interfaces built over UIKit

Mac UI
MASPreferences Preferences window for Mac Apps

Images
FLAnimatedImage Performant GIFs for iOS
GPUImage GPU based image and video processing for iOS
FastImageCache iOS library for quickly displaying images while scrolling
SDWebImage Asynchronous image downloader with cache support for iOS

Animation
Pop Extensible animation library for iOS and Mac
JazzHands Key-frame based animation for iOS

Logging
CocoaLumberjack Better logging for iOS and Mac

Model Programming
Mantle Simple model layer library for Mac and iOS
Realm A new mobile-first database

Network Programming
GCDWebServer HTTP server for iOS and Mac
CocoaAsyncSocket TCP/IP socket server and consumer for iOS and Mac
Restkit Transform REST API calls into models (including Core Data) for iOS and OSX
AFNetworking "The" networking framework for iOS and OSX
AlamoFire A networking framework for Swift, written by the creator of AFNetworking
Reachability Reachability class for iOS and OSX

Audio
AFSoundManager Easier local and streaming audio playback for iOS

Localization
TTTLocalizedPluralString Much easier pluralization for NSLocalizedString
InflectorKit Built in pluralization rules for a bunch of languages
FormatterKit stringWithFormat that localizes different types of data

Programming Helpers
Dollar.Swift Functional programming helpers for Swift
ObjectiveSugar Functional programming helpers for iOS
Reactive Cocoa Functional Reactive Programming for iOS and Mac
libextobjc Cocoa library to extend Objective-C
PromiseKit Better asynchronousa programming with Promises for iOS (Obj-C and Swift)
DateTools Helping out with date and times
KVOController Block based, easier, safer, KVO
PureLayout Better programmatic Auto-Layout for iOS and Mac
SwiftyJSON Handle JSON in Swift with ease

Testing
Quick Behavior-Driven Development testing framework for Obj-C and Swift
Specta Light weight TDD/BDD framework for Obj-C, built on top of XCTest
Expecta A nice matching framework to make your tests more readable
OHHTTPStubs Stubs for web request unit testing
OCMock Mock objects for Objective-C unit testing

Misc
Sparkle An updating framework for Mac

Xcode Plugins
All plugins should be installed with Alcatraz, the package manager for Xcode.
FuzzyAutocomplete Fuzzy pattern matching for Xcode's autocomplete
Lin-Xcode5 Localization Manager for Xcode
LLDB-Quicklook Quicklook for a variety of additional types
KFCocoaPodsPlugin Xcode Plugin for CocoaPods with pod commands/console output, user notifications & code completion
KSImageNamed Xcode Plugin to autocompleteimageNamed:
method calls
VVDocumentor Get help writing documentation in Xcode

Tools & Developer Apps
Synx Sync project folders to Xcode groups
Dash A wonderful documentation app for the Mac with integration everywhere
CoocaPods A fantastic dependancy management system for iOS and Mac
Crashlytics Crash reporting and beta testing for iOS and Mac (and others)

Websites and Resources
NSHipster NSHipster is a journal of the overlooked bits in Objective-C, Swift, and Cocoa Updated weekly.
ObjC.io A periodical about best practices and advanced techniques in Objective-C
Ray Wenderlich Programming Tutorials focused on iOS
inessential Brent Simmons' blog (Programmer for Q Branch)
mjtsai Michael Tsai's blog
NSBlog Mike Ash's blog
Cocoa Blog brought to you by the iOS developers at Tumblr
Cocoa Controls iOS UI controls
Pttrns iOS design inspiration