I've got 3 blogs so I have a few different answers as to each.
-Work Blog 1 is for our insurance agency... traffic has been driven by facebook, twitter and getting on the front page of digg, stumbleupon, or reddit. The most recent was a post for something to the effect of save a bundle on insurance for your mobile devices (ipad, iphone, etc). It was something that hadn't been talked about much but ended up getting us about 2,000 visitors through stumbleupon. Comes down to content I guess because we got picked up by a few big Mac/iOS sites.
-Work Blog 2 is for an insurance agnecy start up company... we help people get their agency started. This is all done through newsletters and google since we're targeted a specific group of people (some known).
-Personal Blog... place where I talk about all kinds of stuff but mostly home theater, computers, networks, and movies. Most of this traffic comes from blog comments as well as co-hosting a well known home theater podcast a few times.