Cinder is a new C++ library for creative applications. It is free, open-source, and cross-platform (Windows, Mac, iPhone/iPad, but no Linux). Think of it as Processing, but in C++.
Cinder offers classes for image processing, matrix, quaternion, spline and vector math, but also more general stuff like XML, HTTP, IO, and 2D Graphics.
The more generic stuff is implemented via third-party libraries, such as TinyXML, Cairo, AntTweakBar (a simple GUI), Boost (smart pointers and threads) and system libraries (QuickTime, Cocoa, DirectAudio, OpenGL) – certainly an ambitious range of technologies and uses.
Their examples are impressive, especially some of the demos by Robert Hodgin (flight404):
Cymatic Ferrofluid by flight404 (be sure to watch the videos).
Robert Hodgin has also created a very nice Cinder tutorial, which guides you through the creation of a quite spectacular particle effect.
Finally, it should be noted the openFrameworks offers related functionality, also based on C++.