Volumouse is a flexible tool which allows you to control the volume of your speakers by spinning the mouse wheel.
Except, of course, it's not quite as simple as that. Sometimes you'll want to use the mouse wheel for other things, and so Volumouse allows you to construct complex rules which define exactly when you'd prefer it to tweak your system volume: when the left mouse button is down, say, or when you're press the Alt or Ctrl keys.
Need even more configurability? By default Volumouse will adjust the volume for all channels, but if you want to tweak one in particular then that's easy too. And you can also choose the number of steps each mouse wheel roll will move the volume up or down (more = faster movement).
A new ability in this beta is to change the volume for a particular application only (you'll need to click the More option next to the rule to see that).
And if this still isn't quite enough, then you can also spin the mouse wheel to change a few other things, including window transparency. So just hold down Alt, say, and spin the mouse wheel, and with Volumouse running in the background then you're able to make the current window more (or less) transparent.