kJams

That is actually all I meant by file compatibility, the ability to play the files in the player. As I said, actually making the game work is a serperate matter. Many people have ultrastar collections in the home market, and might want to play them in your player. That said, here is a link to the OSX...

Actually, there is a port for OSX, albeit for intel machines only. Since what is at issue initially is just file compatibility, I should think that would offer a very good head start, since all it consists of is an audio file and a separate text file with vocals and timing cues. As for actually havi...

Ultrastar support would be a nice added feature, both for the occasional person with large Ultrastar collections, as well as for allowing karaoke "competitions."
Also, the source code for both Ultrastar and Ultrastar Deluxe are open source.