Is there something like nvenc for free codecs (VP8/9)?
There is a hardware-accelerated VP8 encoder recently released by the University of Milan. Source and standalone binary for 64-bit Linux and  compute 3.5, 5.0 and 5.2 capable NVidia GPU cards available at https://github.com/Italtel-Unimi/libvpx

They say they have integrated the code into libav libs i.e. FFmpeg, but this hasn't been pushed into the FFmpeg master branch.