If your Mac's speakers leave a little bit to be desired, Boom will boost your volume past its maximum point, making that sound easier to hear.
We've talked about Boom before, and its really easy to use. Just start it up and click on its menu bar icon when you want to raise the volume. Its only downside was its price tag, but today it's been discounted down to $5.49. We'd still love to see it a dollar or two cheaper, but we'll take what we can get — plus it has a free trial if you want to try it out before you buy.