LV2 is an open standard for plugins and matching host applications, mainly targeted at audio processing and generation. LV2 is a simple but extensible successor of LADSPA, intended to address the limitations of LADSPA which many applications have outgrown.
LADSPA
LADSPA is an acronym for Linux Audio Developers Simple Plugin API. It is a standard for handling filters and effects, licensed under the GNU LGPL. It was originally designed for Linux through consensus on the Linux Audio Developers Mailing List, but works on a variety of other platforms. It is used in many free audio software projects and there is a wide range of LADSPA plugins available.
DSSI
DSSI (“dizzy”) is a virtual instrument (software synthesizer) plugin architecture for use by music sequencer applications. It was designed for applications running under Linux, although there is nothing specific to Linux in the interface itself. It is distributed under the terms of a combination of GNU Lesser General Public License and some BSD licenses, all of which are free software licences. Check Wikipedia and this website for more info and dssi applications.