Proxima Controller is a MIDI controller application that allows you to use the mouse and computer keyboard to play external MIDI devices such as synthesizers and tone generators. It is also possible to play internal devices if they are designed to respond to MIDI messages (for example, the Microsoft GS Wavetable SW Synth). This application makes no sounds of its own and instead relies on the MIDI devices it is controlling to produce sound. It supports bank and patch changes, transmits on MIDI channels 1-16, includes pitch and modulation sliders, and has five octaves of control.
Proxima Controller is simple, easy to use, and works well for controlling MIDI hardware. It has been around for ten years and has always been freeware.