Today, 3 new apps have been released on the Mac App Store.
SkyEater OO Free is a free monophonic synthesizer. You can play melody with keyboard on your mac. In Pro version, you can edit many parameters (Oscillator types, attack time, release time, glide time, and echo effect settings) to adjust sound character.
If you want to play chords, try SkyEater POLY. SkyEater POLY is a polyphonic synthesizer.