Libvpx is part of the WebM project, the AV container format backed by Google for royalty-free open video compression on the web. The libvpx library is the VP8/VP8 codec SDK. With the 1.3.0 "Forest" release that happened on 15 November, the VP9 video codec is now supported in a backwards compatible manner.
From the libvpx release notes, "This release introduces the VP9 codec in a backward-compatible way. All existing users of VP8 can continue to use the library without modification. However, some VP8 options do not map to VP9 in the same manner. The VP9 encoder in this release is not feature complete. Users interested in the encoder are advised to use the git master branch and discuss issues on libvpx mailing lists."
The 1.3.0 also has numerous enhancements, a lossless compression mode, vpxdec and vpxenc support scaling, various speed optimizations, and lots of bug-fixes.