The Raspberry Pi and the Teensy work well together
Teensy audio adapters stacked together
The BELA: good solution for low latency audio
SHARC Audio Module (Analog Devices)
Stanford DSP Shield (Fixed Point :(( )
FPGA Boards (here Atlys Spartan-6)
usbMIDI.sendNoteOn(note, velocity, channel);
usbMIDI.sendNoteOff(note, velocity, channel);
usbMIDI.sendAfterTouchPoly(note, pressure, channel);
usbMIDI.sendControlChange(control, value, channel);
usbMIDI.sendProgramChange(program, channel);
usbMIDI.sendAfterTouch(pressure, channel);
usbMIDI.sendPitchBend(value, channel);
Here from the Teensy USB Library