libmatroska

In short, matroska is a new Audio/Video container file format. It is an advanced and full featured format.

Advanced because it is based on EBML, a kind of XML equivalent, that allows infinite extensibility of the format. And full featured because it includes precise seeking, any audio/video/subtitle codec support including VCM/ACM/DirectShow compatibility, timecode based format, complex frame dependencies, chaptering, internationalisation, error protection, tagging, file attachement, control track (to be defined), menu (to be defined), etc. All these features are not yet implemented but already defined in the format.