Sometimes some youtube clips, movies or podcast have their audio recorded really low. My Macbooks' sound is not really strong, so when I listen to such a video I struggle to catch the speech - especially when I'm in the shower.
Global Delight has created Boom. It costs $4.99. A software solution to such a problem - it can boost the volume up to 400% and has a system-wide equalizer:
Here's an indian presenting it: http://www.youtube.com/watch?v=MMiv-qHHeNw&feature=player_embedded#at=197.