Our website provides a free download of i-Synth 1.2 for Mac. The application belongs to Audio & Video Tools.
From the developer:
iSynth is a standalone virtual instrument distributed in the US by Cycling '74 and features: OMS support for real-time MIDI control Up to six virtual instruments Built-in audio mixer Integrated step sequencers for each instrument Pattern manager lets you combine sequences into performances Built-in arpeggiator Built-in chorus, phaser, flanger, overdrive, three-band EQ, lowpass, ring modulator, and gate effects processors
You may want to check out more Mac applications, such as Massive Audio Compressor, which might be similar to i-Synth.