SampliTron is one of the most popular Windows apps I’ve written. Although it’s fairly simple, it’s pretty powerful. It’s a virtual sampler that lets you load a .wav file and scale it across the entire keyboard, with that keyboard playable via either the computer keyboard or an external MIDI controller.
Before today it was a commercial app with a demo version, and the full version was $15. Over its lifetime it’s been downloaded more than 40,000 times and has sold a few dozen copies.