Mac: iTunes is pretty much the default media player for most Mac users, but if you wish you could change its look, Bowtie lets you customise the iTunes Mini Player with a number of themes ranging from minimalist to flamboyant.
If you prefer the iTunes Mini Player out of the way or transparent, you can set Bowtie to be a tiny translucent player that sits on the desktop and displays the current song only. You can just as easily make Bowtie look like an LP emerging from its sleeve. Bowtie also posts your tracks to Last.fm so friends can see what you're listening to.
Depending on your tastes, Bowtie can add personalisation to your system, or keep iTunes out of the way entirely. Best of all, it's completely free, and available through the Mac App Store.