I use the fall protection system manufactured by Pray Inc.
It's the simplest system I have ever found... All you do is get up on the roof and PRAY that you don't fall.

I'm suprised Glass didn't beat me to it!
Seriously though the answer depends on the type of roof. For steep shingle roofs I use a product called "Bucket O' Saftey". No this one isn't a joke! It is a bucket that comes with everything you need, bracket, rope, shockabsorber, and harness. I've never found a single one of these to be comfortable. Infact in most cases I found them to be clumsy and have even tripped a few times because of them. Me? I prefer a chicken ladder, scaffolding and roof jacks.
LOL every time we talk about fall harnesses I get the picture in my mind of this guy roofing a 14/12 section of a roof holding onto a rope with one hand and nailing the shingles with the other. I wish I had my camera with me that day!