Simplesynth is described as 'SimpleSynth is a small and fast synth, perfect for when you want to try out a musical idea quickly. It's got a complete general MIDI instrument set built in, and it can load Soundfonts and DLS files' and is an app in the audio & music category. There are five alternatives to Simplesynth for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Android Tablet. The best alternative is Synthesia, which is free. Other great apps like Simplesynth are Linthesia, PianoBooster, Musicope and Pianu.
Simplesynth is mainly a Piano Trainer but alternatives to it may also be Music Games. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Simplesynth.
Piano Booster lets you practice and play the piano alongside a scrolling music stave. It's an open-source, cross-platform tool that works with MIDI keyboards as input. Play along and receive an accuracy reading to let you know how you're doing.