Linux Audio Developer’s Simple Plugin API (LADSPA)
LADSPA is a standard that allows software audio processors and effects to be plugged into a wide range of audio synthesis and recording packages.
For instance, it allows a developer to write a reverb program and bundle it into a LADSPA “plugin library.” Ordinary users can then use this reverb within any LADSPA-friendly audio application. Most major audio applications on Linux support LADSPA.
If you are a developer and want to jump straight into the code, you may want to look at the LADSPA header file or download the LADSPA SDK.
For a well-written introduction to LADSPA, see Dave Phillips’ article on Oriellynet.