Memes

via imgur, the above are all non-rectangular alpha-transparency-having themes (or "faces") used by Panic's Audion from 1999 to 2004.

There is a sad story about how it almost became iTunes, but because the developers were already in talks with AOL they tried to invite AOL to their Apple meeting and... apparently AOL couldn't fit it in their busy schedule, so, iTunes was instead based on an inferior player distributed by Casady & Greene called SoundJam which looked like this:

edit: TIL that in 2021 Panic released "a stripped-down version of Audion for modern macOS to view these faces". Unfortunately all of the screenshots in the linked directory appear to be 404 now, but the faces themselves do appear to be downloadable. (Also unfortunately, there still doesn't appear to be a player that can use them without running a proprietary OS...)