To keep the free features free, WordPress.com sometimes displays advertising on member’s blogs. See Point 10 in the Terms of Service. http://en.wordpress.com/tos/ that you agreed to when you signed up.
FWIW-as a logged out user, I saw one ad at the bottom of your post from the 22nd of March; you may have noticed the “About these ads” link just above the ad, which, if you had clicked on it, would have given you information about advertising. As soon as I logged in, the ad disappeared.