Native Instruments Reaktor is, famously, a complete virtual modular synth construction set. It began as one of the first truly viable virtual analog synths in the mid-1990s (though Seer Systems Reality, developed by none other than Dave Smith, is generally credited as the first).
Reaktor 6 Player may not provide as much extensive ability to create your own instruments, but it amounts to an easy-to-use DSP lab that's an ideal introduction to modular synthesis - all without investing in a single piece of hardware. Think of it as a "lite" version of Reaktor, though that's something of a term of art, as you could take months to exhaust what Reaktor 6 Player offers.
From NI's site: "You also get the free Reaktor Factory Selection, which features more than 400 presets for four powerful VST instruments: Lazerbass, Space Drone, Carbon 2, and Newscool. Simply load an instrument and start playing."
In addition, it powers the free (but separately downloaded) Blocks Wired and Mikro Prism soft synths. Reaktor 6 Player is likely the most software synthesis power that can be had for a price of $0.00. To get it, hit NI's download and info page here.