gst-av is a GStreamer plug-in to provide support for libav (formerly FFmpeg), it is similar to gst-ffmpeg, but without GStreamer politics, which means all libav plugins are supported, even if there are native GStreamer alternatives; VP8, MP3, Ogg, Vorbis, AAC, etc.
In addition, it is much simpler (2654 vs 16575 LOC), has better performance, and has a bit of extra features (such as less latency), and doesn’t use deprecated API’s. In a previous post I measured exactly how much improvement compared to gst-ffmpeg there is; it’s not much, but it’s some.
IOW; it’s possible that gst-av is the only GStreamer codec plug-in you would ever need :)
Compared to the previous version, now there’s video decoding (H.263, H.264, MPEG-4, WMV and VP8), and video encoding (H.263 and H.264), plus a fix for audio decoding with playbin, and better integration with the official FLAC parser.
BTW. It works both for libav and FFmpeg.
There’s now a project page hosted in Google Code where you can find all the juicy stuff.