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 VST 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 for MacOS and Linux, and VST plugin for Linux.

See the official webpage also.