I've been working with a NetFPGA 10G board. It's quite cheap for academic research, but not for hobbyists. It's also rather old now. The project comes with open source HDL, but I've had immense trouble getting it to work. You need a special license from Xilinx just to get started (ethernet core). I wish I had chosen something else. I tried a Lattice ECP5 Versa board, but you need to sign a NDA just to get the ethernet documentation, which of course is impossible.