I’ve compiled a list of six popular iPhone apps and the iOS Open Source Software they use.

I’ve also linked to the original repositories in case you want to check them out and start contributing to them. Projects that seem inactive, outdated or no longer maintained have been excluded from the list.

Instagram

AFNetworking: iOS and OS X networking framework.

Appirater: Reminds your iPhone app’s users to review the app.

asi-http-request: Easy to use CFNetwork wrapper for HTTP requests, Objective-C, Mac OS X and iPhone.

CocoaHTTPServer: Small, lightweight, embeddable HTTP server for Mac OS X or iOS applications.

Cocoa Lumberjack: Fast & simple, yet powerful & flexible logging framework for Mac and iOS.

MBProgressHUD: Drop-in class that displays a translucent HUD with an indicator and/or labels.

PLCrashReporter (Github mirror): In-process crash reporting framework.

QSUtilities: Loose collection/library of utilities, controls and other helpers for iOS (iPhone, iPod Touch, iPad) development.

SocketRocket: A conforming Objective-C WebSocket client library.

XBImageFilters: OpenGL ES 2-based image and real-time camera filters for iOS.

Foursquare

Facebook SDK for iOS: Integrate with Facebook, help build engaging social apps, and get more installs.

FSNetworking: Foursquare iOS networking library.

kingpin: Drop-in MapKit/MKAnnotation pin clustering library for MKMapView on iOS.

AFNetworking: iOS and OS X networking framework.

SKBounceAnimation: CAKeyframeAnimation subclass that lets you quickly and easily set a number of bounces, and start and end values, and creates an animation for you.

DB5: App Configuration via Plist.

LinkedIn

BlocksKit: The Objective-C block utilities you always wish you had.

SDWebImage: Asynchronous image downloader with cache support with an UIImageView category.

DTCOreText: Methods to allow using HTML code with CoreText.

Shazam

AudioStreamer: Streaming audio player class (AudioStreamer) for Mac OS X and iPhone.

ColorArt: iTunes 11-style color matching code.

objc-geohash: Objective-C GeoHash Library. Get hashes by longitude and latitude.

TPCircularBuffer: Simple, fast circular buffer implementation.

FormatterKit: stringWithFormat: for the sophisticated hacker set.

for the sophisticated hacker set. UIView+Glow: Category on UIView that adds support for making views glow.

WEbViewJavascriptBridge: iOS / OSX bridge for sending messages between Obj-C and JavaScript in WebViews.

Skype

AFNetworking: iOS and OS X networking framework.

Hockey SDK: Official iOS SDK for the HockeyApp service.

PLCrashReporter (Github mirror): In-process crash reporting framework.

TTTAttributedLabel: Drop-in replacement for UILabel that supports attributes, data detectors, links, and more.

SDWebImage: Asynchronous image downloader with cache support with an UIImageView category.

Cocoa Lumberjack: Fast & simple, yet powerful & flexible logging framework for Mac and iOS

MWPhotoBrowser: A simple iOS photo browser with optional grid view, captions and selections.

QSUtilities: Loose collection/library of utilities, controls and other helpers for iOS (iPhone, iPod Touch, iPad) development.

BlocksKit: The Objective-C block utilities you always wish you had.

Spotify