Imwheel makes the wheel of your Intellimouse (and other wheel and stick mice) work in Linux/X11 to scroll windows up and down, or send keys to programs. It runs in the background as a daemon and requires little reconfiguration of the XWindows setup. 4 or more button mice and Alps Glidepad 'Taps' may also be used. imwheel includes a modified gpm for an alternate method of wheel input.
Re: What does this provide?
This program extends the capability of the mouse wheel, and other extra buttons (which most apps still don't recognize, like the side buttons or a left/right wheel). It allows them to act the same as when it isn't running, or to change the behaviour completely when you configure it to do so. It also can change behaviour based on modifiers, which not all apps that recognize the wheel do. Mozilla does, yes, but things like xmms don't. So while you do have "basic" functionality in most X apps these days, you still don't get that extra bit of configuration that can increase your wheel usefulness many times more.