Carla is a fully-featured audio plugin host, with support for many audio drivers and plugin formats.
It's open source and licensed under the GNU General Public License, version 2 or later.

• LADSPA, DSSI, LV2 and VST2, VST3 and AU plugin formats
• SF2/3 and SFZ sound banks
• Internal audio and midi file player
• Automation of plugin parameters via MIDI CC
• Remote control over OSC
• Rack and Patchbay processing modes, plus Single and Multi-Client if using JACK
• Native audio drivers (ALSA, DirectSound, CoreAudio, etc) and JACK

In experimental phase / work in progress:

• Export any Carla loadable plugin or sound bank as an LV2 plugin
• Plugin bridge support (such as running 32bit plugins on a 64bit Carla, or Windows plugins on Linux)
• Run JACK applications as audio plugins
• Transport controls, sync with JACK Transport or Ableton Link

Carla is also available as an LV2 plugin and VST2 plugin.

See the official webpage also.