Really great project and nice collection !
There also are some non-open source but still hackable synth out there, such as the Korg NTS-1. It's not exactly DIY but a "mount it yourself kit" and Korg maintains a C and C++ SDK for oscillo and fx.
Funny part with the x0xb0x, the creator mentioned that the x0xb0x is free as "Free your mind and your ass will follow", the funkiest open source clause