Windows Media Player has an interesting option that will let you open files in the Mini Player mode instead of the full player window. This is especially useful for opening music files where you really don't need the full-size window. To set this option, go to the Options page (linked off the Now Playing button) and look at the first tab, which should be "Player". You'll see a textbox titled "Start the mini Player for file names that contain this text". The default value in this field is "voiceatt.wav", obviously designed to use the mini mode for voicemail attachments. If you want to make all mp3 files open in the mini player, just type .mp3 into the window:

Or if you have all your music stored on a single drive like I do, you can type the drive letter, or even the folder path to your music collection: 

If you want every single file to always open in the mini player, just type a single colon : character into the field. This doesn't work quite as well for videos, though...

Now when you double-click on a file in Windows Explorer, it will open in the mini player by default. On to the screenshots! Windows Media Player 11 on XP (Mini mode)

Windows Media Player 11 on Vista (Mini mode)

The Vista version is definitely cool with the transparency, but I almost like the XP version better.