There are a ton of Mac volume controllers out there. CoverSutra is one of the best that we have seen, however, if simplistic and free is what you like, then why not try iTunesVolume. Just as the name implies, you are able to control the volume of the iTunes application with this small app.
iTunesVolume definitely takes the Steve Jobs "one button is good enough for everything" approach. The slider controls everything -- you can move it to control volume in iTunes or hold the button down to get a pop-up menu with extra options.
iTunesVolume is a free 2 MB download from the developer's website.