Anything that asks me, however nicely, to buy their book / like them on Facebook / check out their blog (sorry) / whatever. I don't mind general Tweets doing those things (though again, I'll unfollow if all you Tweet is ads) but being targeted individually? Nah. Gets my goat.
Grrrr.... My brother-in-law sitting next to me on the porch while I try to get writing done. Bonus pet-peeve.... When he does the same thing while letting his dog lick his fresh surgical wound for 30 fucking minutes!