Carla is a fully-featured modular audio plugin host, with support for many audio drivers and plugin formats.
It has some nice features like transport control, automation of parameters via MIDI CC and remote control over OSC.

Carla currently supports LADSPA (including LRDF), DSSI, LV2, VST2, VST3 and AU plugin formats, plus SF2 and SFZ file support.
It uses JACK as the default and preferred audio driver but also supports native drivers like ALSA, DirectSound or CoreAudio.

There are 4 types of engine processing:

• Single-client: (JACK driver only)
  Same as Multi-client, except that all JACK ports belong to a single master client.
  This is needed when a setup doesn’t support multi-client JACK apps, such as LADISH.
• Multi-client: (JACK driver only)
  Every single plugin is exposed as a new JACK client. Audio and MIDI ports are registered as needed.
• Rack:
  Plugins are processed in order, from top to bottom.
  Plugins with non-stereo audio channels are not supported, but a forced-stereo option is available for Mono ones.
• Patchbay:
  Modular patchbay mode, just like in JACK Multi-client and many other modular applications.
  Every plugin gets its own canvas group and ports allowing you to interconnect plugin audio and MIDI.