Already acknowledged as one of the best free plugin synths in the market, the OB-Xd is now being looked after by discoDSP. The company has said that it plans to "make it better", while also pledging that it will remain a freeware product.
The synth takes inspiration from the Oberheim OB-X, OB-Xa and OB-8 synths, though it goes beyond them in certain areas.
If you don't already own the OB-Xd, we'd recommend that you grab it from the discoDSP website. It's available for PC, Mac and Linux.