DrumPads is an app with 12 virtual drum pads that lets you use the keyboard, mouse, touchscreen, or an external MIDI device to play any of 100 high-quality drum sounds.
DrumPads for Windows. Looks identical on OSX.
It started as a webOS app. It was later ported to Ubuntu and sold via the Ubuntu Store. Then ported to Windows and ended up becoming my most popular app with 330,000 downloads of the free version. I spent some time porting it to OSX a while back, but never had a version in the App Store.
Now I do.