Google is sending me the most traffic. After that, at about 1/4 the rate of Google, it's Technorati. Other search engines are way behind. I'm impressed; at least for my topic area, Technorati seems to be a very popular tool.
I had also started doing del.icio.us tags after reading that it was driving more traffic to ParticleTree than Google. Didn't do anything for me; I think I got one referer over all.
A new player is stumbleupon. Still a small amount of traffic, but steady.