I have a Yahoo email account. Just recently I noticed that when I check my emails, the first thing I see in my list is an advertisement. It can't be deleted unless I delete all my messages. Fortunately I don't use that account very much, because every time I check the emails I get annoyed at the intrusion into MY email account.
OK, I know that Yahoo is providing the service for free, but seriously, do they have to put the ad right in my face? It's so much easier to ignore when it's off to the right side of the page. I guess they figured that out.