Coming in a close second is email touting "hot stocks" to buy. This I blame on making the mistake of getting an account on Motley Fool. And it may also be due to my online brokerages selling my email address. I don't own much stock, but thanks to changes in 401k and stock options at a Microsoft and my current company, I've had accounts online with 6 different brokerages.
And I blame Matchmaker.com for a recent spike in spam sent to an email address which I only use for personals ads. That site is a crock - they advertise a free service, but as soon as you sign up they start sending bait-and-switch email claiming someone has expressed interest in you. You log in only to discover that (a) you can't reply to this "interest" unless you buy a "Gold Account". And when you view the profile of the person who is allegedly interested, they turn out to be way out of your stated geographical range, out of your age range, and not a match at all. I have closed my account, but the damage is done, the spam continues.