Version 2.4 of the Vamp plugin SDK is now available. http://www.vamp-plugins.org/
Vamp is a plugin API for audio analysis and feature extraction plugins written in C or C++. Its SDK features an easy-to-use set of C++ classes for plugin and host developers, a reference host implementation, example plugins, and documentation. It is supported across Linux, OS/X, and Windows.
A documentation guide to writing plugins using the Vamp SDK can be found at http://www.vamp-plugins.org/guide.pdf
Version 2.4 is a maintenance and bugfix release. The main addition is a simple FFT implementation for plugins to use. For more details, see the changelog athttp://code.soundsoftware.ac.uk/projects/vamp-plugin-sdk/repository/entry/CHANGELOG