Buy Humfrees Rack Isolation Tabs | Rackmount Accessories | Musician's Friend
These things work like a charm - from personal experience.
Most electronic (digital and analog) devices connect their internal ground wiring to the metal chassis or frame. In a rack, our gear tends to touch each other and rub up against each other. Combine this with the 3rd prongs on each device grounding to the power strip, and each audio cable having a ground, and you have a dozen or so paths to ground, which causes ground loops (buzzing, hum)
I'm paraphrasing, here is a full article from our own Ethan Winer on the subject:
Kill Studio Hum and Buzz at the Source