First off, most marketing emails are annoying. Having said that, it's amazing at how many companies can't get their act in gear and send emails that are encoded and formatted properly. I suspect many of these companies hire some email-marketing company to do the dirty work for them--and it surprises me more that the "professionals" can't get it right.
Companies who totally screw it up: Crucial, Antec (I can't even make out what they're yammering about), Alienware and... hrm... I can't think of the other ones that annoy me. I'll have to update this entry later.