Due to icestorm's timing, it is now the FPGA family with most mature open flow support. They're also cheap and have cheap OSHW development boards, which helps.
Besides iCE40 (project icestorm), there's also ECP5 (project trellis) and quicklogic eFPGA (support provided by vendor itself!), all in good shape.
Then there's some more, like GW1N (project apicula) and Xilinx family 7 (project x-ray), in a partially-working state.
