VMPK - Virtual Piano Keyboard
VMPK, or Virtual Piano Keyboard, is a MIDI events generator, and reciever, that allows you to play MIDI notes with your keyboard or mouse. You can also create, unlimited, custom controls to send out and recieve MIDI data. This can be useful if you want to control hardware parameters, via MIDI, from your computer.
A very useful feature of VMPK is it's ability to grab keyboard input while it's window is not in focus. This means you can open up a connected synth and tweak its settings with your mouse while continuing to send keyboard input to it from VMPK, even while VMPK's window is not in focus. To enable this feature go to Edit > Preferences and enable 'Grab Computer Keyboard'.