TrackBack Spam. I get about 80 of these a day.

Quoting TrackBack Spam: Abuse and Prevention , Elie Bursztein, Peifung E. Lam, John C. Mitchell, Stanford University, 2008:

“The spam rate [for our fake Wordpress blog] peaked in November 2007, with around 90,000 spams per day. ... During the final phase, we measured between 1000 to 2000 unique IP addresses per day, all from Russia. ... All the URLs in the TrackBack spam we examined pointed to malicious web sites that tried to lure the user into downloading the malware TrojanDownloader:Win32/Zlob.gen!dll. ... The domain used to host the malware changed over time, but the listed owner of all domains was the same, Igor Palki. The registered address for all the malware sites was a small town in Russia, Nijnii Novgorod.”

From http://www.mail-archive.com/funsec@linuxbox.org/msg03346.html :

40 million spams emails sent
0.12% click through rate = 48,000 click throughs
Click through to sale ratio 1/200
Total sales = 240
Total sales revenue = $37,440

Spammer commission rate = 50%
Total spammer income = $18,720

Running costs:
4 day botnet rental     $6,800
Buying email addresses  $4,000
Bullet proof hosting    $230
Total running costs     $11,030

Net profit              $7,690