One flock-to is Lattice ICE40 series. Decent support from Yosys for hobby stuff at least. Possibly Go-win. You can develop for many of the affordable FPGAs with free versions of the proprietary tools. As for "closed architecture" it depends what you mean, the architecture of all FPGAs that I know of is documented, the tools show you how your design was mapped onto the hardware. The proprietary stuff includes the timing model that drives the static timing analysis and timing-aware place and route.
Comments
One flock-to is Lattice ICE40 series. Decent support from Yosys for hobby stuff at least. Possibly Go-win. You can develop for many of the affordable FPGAs with free versions of the proprietary tools. As for "closed architecture" it depends what you mean, the architecture of all FPGAs that I know of is documented, the tools show you how your design was mapped onto the hardware. The proprietary stuff includes the timing model that drives the static timing analysis and timing-aware place and route.