A couple before/after videos with helpful commentary:

Mastering houses with before/after samples of their work:

• http://mercurymastering.com/#ourwork
• http://www.sageaudio.com/
  • Summary of services included: http://www.sageaudio.com/cd-mastering.php
• http://www.atomicmastering.com/index.html#section-four
  • Summary of services included: http://www.atomicmastering.com/index.html#section-three