Flickr Stats: http://blog.flickr.com/en/2007/12/13/stats-stats-baby/
I’m surprised nobody has mentioned this yet - at least, as far as my friends list. I saw it come through on Daring Fireball.
Anyhow, you can request that stats be enabled for your Pro Flickr account, and you can then see referrers and stuff coming in.
Woo stats!