Vamp is an easy system to develop plugins for. It has a standard cross-platform SDK which includes API documentation, example plugins, ready-to-use C++ base classes, the C API header, and a test host.
Vamp plugins use a C binary interface for the greatest level of binary compatibility. However, plugins and hosts are strongly encouraged to make use of the set of C++ base classes provided in the SDK.
Alternatively, you can also now make Vamp plugins in Python using VamPy.
The entire SDK is published under a very permissive BSD-style license. You are encouraged to copy from it wholesale, whether developing open-source or proprietary plugin or host software.
Vamp plugin and host development may be discussed on the vamp-devel list and the Vamp plugins forum.