Keyboard on personal computer is ideal for emulation piano and making a digital organ.
What about mouse?
There are some issues to solve, but it is doable. I wrote a Java theremin. Available to try out if you have Java 8 running. Consider this a beta test version--please write back if you have suggestions! Contact info is on website. http://adonax.com/main/musicapps.htm
Main issues that I've come across:
All of these have been confronted in the theremin implementation I wrote. Successfully? TBD.
Check out Serenade III from the Native Instruments Reaktor User Library (website).
It is "demoable" in Reaktor Player. It's a patch ("ensemble") for Reaktor.