JUCE is an extensive, mature, cross-platform C++ toolkit
Overview
JUCE is a wide-ranging C++ class library for building rich cross-platform applications and plugins for all the major operating systems.
Having been actively developed since 2003, it is used by thousands of developers around the world, and JUCE-based apps are used by millions of people every day.
Key Features
Hundreds of classes covering a vast range of tasks from high-level user-interface handling, graphics and audio, right down to low-level collections, networking, strings, etc.
Supported platforms are OSX, Windows, Linux, iOS and Android.
A modular architecture means that you can easily link to a subset of the library, and the Introjucer project management tool makes it a breeze to maintain cross-platform projects.
What Makes it Unique?
There are lots of practical reasons to use JUCE, but the most common compliment I get is purely aesthetic: "The code's just really nice!"
Open-source projects are often horrifying quagmires of ugly code that you reluctantly work with because they fulfil a purpose.. JUCE is different: I don't want people to use the library just because it does the job that's required – I want people to use it because they enjoy working with it!