Boost - CPP Library

Boost provides free peer-reviewed portable C++ source libraries. Boost libraries are intended to be widely useful, and usable across a broad spectrum of applications. It supports String, Containers, Streams, Generic programming, Concurrent programming, Math, Memory and lot more.

Website Link : http://www.boost.org/

POCO C++ Libraries - Cross-platform C++ libraries with a network/internet focus

Modern, powerful open source C++ class libraries and frameworks for building network- and internet-based applications that run on desktop, server, mobile and embedded systems. It has a collection of C++ class libraries, conceptually similar to the Java Class Library, the .NET Framework or Apple’s Cocoa.

Website Link : http://pocoproject.org

Pantheios - The C/C++ Logging Sweetspot

A C++ diagnostic logging API that is simple to use, 100% type-safe, generic, infinitely-extensible, atomic, platform-independent, and extremely efficient. And, best of all, it upholds the spirit of C: you only pay for what you use. Pantheios is readily extended to use the existing transport mechanisms of feature-rich diagnostic logging libraries such as ACE, log4cpp, log4cplus, log4cxx.

Website Link : http://www.pantheios.org/

STLSoft - C and C++ Libraries

Robust, Lightweight, Cross-platform, Template Software. STLSoft is a collection of STL and standard library extensions, and facades over operating-system and 3rd-party APIs.

Website Link : http://www.stlsoft.org/

Loki - C++ library of common design patterns and idioms

A C++ library of designs, containing flexible implementations of common design patterns and idioms.

Website Link : http://loki-lib.sourceforge.net/

C++ Btree - Template Library B-tree data structure

C++ B-tree is a template library that implements ordered in-memory containers based on a B-tree data structure. Similar to the STL map, set, multimap, and multiset templates, this library provides btree_map, btree_set, btree_multimap, and btree_multiset.

Website Link : https://code.google.com/p/cpp-btree/