Are there any that tap into hardware encoding routines when they're available on the box?
Are there any besides ffmpeg-compiled-with-libvpx that are open-source? Are there any that are available at no cost?
There are a number of GUI frontends for ffmpeg if that is what you are looking for.
I'm just trying to ascertain to see if there's some other tool out there that encodes into these formats. Despite having lots of decoders (including hardware [1]), it appears encoding support is hard to come by.
Especially versus H.264, where there are many encoders, commercial or otherwise, that produce H.264 streams; VP9 hinges entirely on Google's own code, and even then you have to compile it yourself (or somehow obtain a build).