Ask HN: Good open source hardware project to learn from and contribute to?

I'm willing to learn VHDL or Verilog/SystemVerilog and searching for a project to learn from and later to contribute to. There are almost no projects on GitHub. So are there actually any active and contributor-friendly hardware project?

Also what should be motivation in choice between VHDL, Verilog and SystemVerilog? Are there any recommended books?