Opus is a new free, open and royalty-free audio format that aims to be flexible enough to be used for all kinds of purposes, from VoIP and low-latency uses such as online games to high quality music.
The format was designed through a collaboration between IETF (Internet Engineering Task Force), Skype (Microsoft), Mozilla, Xiph.Org (the creators of Ogg, Vorbis, and Theora), Google, Octasic, and Broadcom. Now it has been standardized by the IETF as RFC 6716.
Opus as it stands is much more flexible than competing audio formats such as MP3, AAC and Vorbis, and in a majority of cases it offers better quality. In addition it is royalty-free and open so it is possible to adopt in any software.
Even though the format is still rather new, it is already being implemented in a number of audio playback, and communication software. It was added to Firefox in Firefox 15, and is likely to become the format of of choice for Skype. The popular VLC Media player and foobar2000 software also support this format. The popular Mumble VoIP software for gaming also supports it.
Perhaps the most important factor though is that is has been adopted as one of the formats of choice for the WebRTC (Web Real Time Communication) API which is a new web standard for building browser based applications for voice chat and other forms of P2P communication without needing browser plugins. This means it will be required to implement it in any browser that supports WebRTC.
Currently though support in popular media players and operating systems is still week. Over time however this is a format to look out for.
