FoxyTunes is a Firefox extension that lets you control a music player from the browser. The idea is not original, but this add-on supports a lot of music players (from Winamp, iTunes to RealPlayer, Windows Media Player, Amarok and foobar2000) and autodetects the current player. You can even use keyboard shortcuts if you don't like to click on small buttons.
If you use Winamp, you can obtain a similar effect by selecting the modern skin and entering the "Windowshade mode", a mini version of Winamp that could remain always on top. Windows Media Player can also be minimized to a small player integrated into the taskbar.
So why would you install this extension? Well, maybe you use a player that doesn't have a mini mode. Or maybe you want to use the search functionality of FoxyTunes. You can find information about the artist, song lyrics, videos and buy songs from iTunes. Foxytunes takes advantage of the new medium by letting you play files linked in a web page.
The interface is customizable and there are skins you can download. If you spend most of your online time in Firefox, it's a good idea to try this extension.