From Hydrogenaudio Knowledgebase
GStreamer is an media framework for Unix. Application such as media players and editors can be built atop GStreamer, which can handle a wide range of audio and movie formats.
Note: List is very incomplete.
Audio: WAV, AIFF, AAC, MP3, (Ogg) Vorbis, FLAC, Speex, Monkey's Audio, TTA, Musepack
Video: Ogg Theora
Containers: AVI, OGM, Matroska