I have a distant cousin who forwards every viral email my way. I've written some careful GMail filters to trash anything he sends that starts with "Fwd." It seems to be working. I haven't heard from him in years.
But sometimes there's a friend you'd rather educate than censor. You spend the better part of an hour researching and delicately wording a rebuttal to his or her nonsense. For years Snopes.com has been an amazing ally in this fight.
But now there's a little more help in the political arena. Media Matters has unveiled Email Checker which will help you reply to the conservative misinformation contained in the most common and most egregious chain and viral emails.
I like the way they word their ready-made responses. They're gentle enough even for your grandma. A typical response contains something like, "If this one were true, I'd be pretty upset - so I decided to see if I could find out for myself..."
That's probably the best approach. Almost makes me want to pull a couple of my cousin's emails out of the trash bin... Nah.