We were fans of the original Boom because it did a great job of boosting the volume on crappy MacBook speakers. The new version, Boom 2, still does that, but it also gives you finer control over your Mac's sound.
Like the previous version of Boom, Boom 2 boosts your volume system-wide. This time around, you can also enhance the sound using built-in equalisers, frequency controls and audio effects. It does a great job of enhancing the audio on the tiny, junky speakers you'll find on smaller MacBooks. It's a bit pricey at $US14.99, but you can check out a free seven day trial from the developer's site below.
Boom 2 ($US14.99)