With Virtual MIDI Keyboard you can trigger MIDI notes using the mouse or the computer keyboard. It has five knobs which send volume, panning, modulation, and two user configurable midi controllers. Another feature of Virtual MIDI Keyboard is that it can send MIDI controller data when you move the mouse while playing the keyboard.
Virtual MIDI Keyboard is freeware.
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http://www.granucon.com/SoftwarePages/Vmk.aspx