CELT
Constrained-Energy Lapped Transform (CELT) Codec. Experimental low-delay wide-band audio codec from the creator of Speex.
It is deprecated in favor of Opus which CELT became a part of.
Unfortunately Opus contains about twice as much code as CELT and takes about twice as much CPU while not exceeding CELT performance for music (for speech Opus suits better, due to SILK).
See e.g. a comparison done under fall 2012: http://www.hydrogenaudio.org/forums/index.php?showtopic=97913&st=0.
For archiving of music, especially for possible playback on resource-constrained devices CELT has thus a value on its own, not necessarily as a part of Opus.
It is unfortunate that standardization of CELT on itself did not complete (https://tools.ietf.org/html/draft-valin-celt-codec-02).