A consortium lead by Matrox - fully named Open Digital Media Consortium. They have defined extensions to the Microsoft's RIFF AVI format to make the format more suitable for professionals (for example, files larger than 1 gigabyte are supported) and to improve the MJPEG support. Microsoft reportedly has cooperated with them.
In some programs I've seen, the OpenDML AVIs are just labelled "OpenDML AVI"; others just speak of "new" AVI format.