Haruhi is a GPL3 host for its plugins written for GNU, using JACK for audio and
ALSA for MIDI. Currently there is only one developed plugin: Mikuru, virtual analog synthesizer.
There are plans for LADSPA support and some MIDI effects.
Haruhi is very young and there are many features to be implemented and many bugs to be found and smashed.
And yes, Haruhi and Mikuru names are from "Suzumiya Haruhi no Yuutsu".