Dental Plan is an older image macro meme originally started on 4chan in 2004, and was resurrected in 2006. It is normally replied to with the Lisa Needs Braces image macro. The Dental Plan meme is also known as Lenny Leonard, which is the name of the character in the macro.
The meme originates from The Simpsons episode "Last Exit to Springfield" (9F15), in which Homer's brain continually repeats "Dental plan! Lisa needs braces!"