One of the neatest parts of OS Xis sometimes controversial Dock is the ability to put "live" applications there. Apple calls them Dock Extras, and includes one for AirPort, one for monitor settings, and one to measure PowerBook battery life.
Another company, On-Core, has released two more Dock-based applications that they call Docklings. On-Coreis two Docklings offer handy controls for system volume and audio CD playback. According to On-Core:
AudioCD Player Dockling - Ver 1.0.0: Add this dockling to your dock and play audio CDis! What could be easier?
Volume Dockling - Ver 1.0.0: Add this dockling to your dock and be able to control the system volume easily.
Both these Docklings are available as "Donationware." You can find more information at the On-Core Web site.