There are some things where a mouse is the perfect input device. There are other things where the keyboard makes more sense. Good software should give you the option, but otherwise good software sometimes doesn't.
I don't know about you, but I find that having to move the mouse to the system track, click on the speaker icon (assuming it's not hidden), and move the mouse a slider every time you want to adjust speaker volume (or to mute the speakers) is a complicated process for what should be a simple task. You can adjust your TV volume by pressing a button on a remote, or the volume on your iPod by touching the control.
Even within a program, it's a pain. You have to click on the program window, find the slider and move it. If you want to mute speakers because you got a phone call, this is awkward.
That's why I find Hotkeycontol XP essential. It runs in the background and you can set up key combinations to adjust or mute volume. Ctrl/+ makes things louder, while Ctrl/- makes it quieter. I also set Ctrl/Backspace to mute. One touch of a keyboard, and I can adjust things. Phone call? Just press Ctrl/Backspace and the speakers are muted.
That's just the beginning. You can also use Hotkeycontrol to open files, shut down your computer, and perform other tasks. For instance, I often need to bring up the calculator. Instead of clicking on "Start," "All Programs," "Accessories," and "Calculator," I hold down the Windows key and press "C." Up comes the calculator. Very handy. And you can use this for any program or folder you want.
It's a nice tool that saves time.