Cakewalk Project (WRK)
Cakewalk by Twelve Tone Systems (later Cakewalk, and now Roland) previously wrote its MIDI sequences as .WRK files.
.WRK files are loosely based on the MIDI file format. They can contain embedded samples which trigger during the sequence. They also contain additional data not found in a standard MIDI file such as track delay, track transpose, track velocity offset, comments and editing time.
The Drumstick open source library can work with .WRK files: https://sourceforge.net/p/drumstick/news/
To be added to this page: a description of the file format taken from the source code of the library.