OctaSine

Free FM synth for Win/Mac on GitHub.

-Four operators with independent parameters such as volume, panning, modulation index, feedback, three different frequency modifiers (ratio, free and fine) and ADSR volume envelope parameters. The operators can be independently switched to white noise mode

-Flexible routing allowing setting the output operator (with some limitations) as well as the percentage of signal that is simply added to the final output, enabling additive synthesis. By default, operator 4 is routed to operator 3, operator 3 to operator 2 and operator 2 to operator 1.

-Master volume and master frequency parameters

-Four LFOs capable of targeting most operator parameters as well as most parameters of lower index LFOs.

-128 voices (using them all simultaneously might consume quite a bit of CPU time though)

-Fully automatable