The referrals they give to other bloggers. I've found wonderful blogs (not all are listed on the right, just the ones I'm compelled to check daily) through bloggers referring other bloggers.
Just this morning, I found Bob's Blogscript through A Day in the Life of an Ambulance Driver who I found through (I think) Nautical Dawn, who I found through both Unix Jedi and Grouchy Old Cripple. Or maybe I found AD through Nothing is Sound who I know I found through Nautical Dawn.
Can't wait to see where Bob is going to lead me next!