Boost is a large collection of free, high quality C++ libraries that cover a broad range of topics. It is often considered a "second standard library" for C++, since many common problems in C++ are solved by using Boost.
From boost.org:
Boost provides free peer-reviewed portable C++ source libraries.
We emphasize libraries that work well with the C++ Standard Library. Boost libraries are intended to be widely useful, and usable across a broad spectrum of applications. The Boost license encourages both commercial and non-commercial use.
Some Boost libraries have even made their way into the C++11 standard library, and some other, such as Boost.Optional and Boost.Variant, will be a part of C++17.
Boost covers most corners of programming. From the boost tag wiki here on Stack Overflow:
It includes libraries for:
- String and text processing
- Containers
- Iterators
- Algorithms
- Function objects and higher-order programming
- Generic Programming
- Template Metaprogramming
- Preprocessor Metaprogramming
- Concurrent Programming
- Math and numerics
- Correctness and testing
- Data structures
- Image processing
- Input/Output
- Inter-language support
- Memory
- Parsing
- Programming Interfaces
- Miscellaneous
- Broken compiler workarounds