I searched and couldn't find a proper thread about this. I thought there would be; feel free to link me if I missed it.
"Omake" simply means bonus item and can refer to a free figurine or anything like that, but in the context of gaming, it usually means extra files on the disc that can be accessed by a PC. Sometimes they will be in a folder labeled "omake". While it was once decently common, I have no idea if modern games ever do it at all.
So... what all games have it?