Common term used to describe the fans of conservative talk show host Rush Limbaugh. It derives from the nature of his show, which allows listeners to call in and espouse their views for a brief amount of time. Rather than waste their precious time telling Rush how much they love him and his views, they simply began saying "Ditto" as shorthand. Thus came the dittoheads.
The term is used as an insult by critics of the show, who think that Limbaugh's fans really are just dittoheads, unthinking parrots who only know how to agree with whatever somebody tells them is true.
It's also a high-octane song from California thrash metal giants Slayer.