Sonic Space Gemma is free VSTi FM synthesizer for Windows.
Here's what the developer has to say about it:
Essentially it is a simple 2-OSC FM synth, but with a flexible panning and delay system. Two wave modulators control the frequency of each independent OSC carrier. Many different waveforms are available for both the modulators and the carriers. Two ADSR envelopes control the AMP OSC curve, while a third envelope is provided to adjust the Pulse Modulation of the frequency. Two Low Pass Filters, one for each channel, have also been introduced together with two delays and an two LFO units. Gemma is velocity sensitive and responds to MIDI messages. Use it to make pads or string-like sounds.
Seamless audio and MIDI switchover between computers