Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
keezy - A colorful soundboard. Play with music.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Figure - Propellerhead creates world-class software products and services that inspire music makers and provide the foundation for a worldwide creative musical community.
ReSlice - Slice your audio with an advanced, rhythmic arpeggiator and FX.
Splice - Music creation, collaboration, and sharing made simple.