Version 1.3 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.
Version 1.3 is a maintenance release, with several bugfixes (almost all of which only affect hosts, not plugins) and no new features.
Changes since 1.2:
-  PluginBufferingAdapter has several important fixes to bugs that could cause incorrect timings or output descriptors to be returned
-  Conversion between real-time and frames has been improved to avoid rounding error in round-trip calculations
-  Plugin lookup no longer relies on non-portable DT_REG
-  The SDK now compiles with gcc 4.3
Plugins and hosts remain binary compatible with those built using the
1.0 version of the SDK.
Chris