I just noticed that the banner ads at the top of Sheri's blog and Tim and Lisa's blog are customized to the content of what they are writing about.

At the top of Sheri's blog there is an ad about chemo therapy and lung cancer because of her writing about Goliath's illness.

At the top of Tim and Lisa's blog is an ad for a Train CD because she wrote about Amtrak, and an ad for a sleep aid because she mentioned what time they fell asleep.

I have a paid blog and therefore don't have any banner ads.