Obxd is a new free
Windows VST plugin synth that's based on – but not a direct copy of – Oberheim's OB-X. It takes the hardware's "sound and behaviour" and then adds new features that are designed to overcome its perceived limitations.
These include a blendable multimode filter, but one thing that hasn't changed is that there are still not effects, so you'll need to bring your own if you want to beef up Obxd's sound.
The synth is available in 32-bit and 64-bit versions and can be downloaded now from the Obxd website.
Update: A Mac version and a folder of AU presets have now been added to the Obxd site too.