I typically just delete them, after shaking my head. If they suggest you've got to send them back to the original sender to tell them you love them, I may take the time to reply, but I never forward such. Anything I do forward, jokes or other meaningful info, I delete all e-mail addresses attached before forwarding.
Dependent upon content, sometimes I will advise the sender that the information is erroneous. Sometimes I take the same approach Brian did, it's totally dependent upon whether the e-mail is something that's going to cause someone harm. Such as e-mail hoaxes suggesting you remove files from your computer that are harmless operating system files. As Brian did, I generally back it up with links to fact, supporting my claims that the original information was in error.
If the original sender takes offense, then I simply tell them they should do their homework before sending such goodies out. And, find some satisfaction in knowing that I have spared several folks on that mailing list the trouble incurred in such matters.
As has been suggested above, one should never send mass mailings, including everyone's e-mail address. Set up a dummy account to send to yourself, address it to "Friends", "Special Friends", "Folks I care about", "My Joke Mail List" or something, then load everyone you want to send to in BCC. If it's a select bunch of friends that you share everything with, setting up a mail group to send is fine, but I often find that we "all", as individuals, have different things that are kind of iffy in our respective lives, and that method will eventually cause you to step on someone's toes. Much easier to set them up in a folder, and include them individually in BCC than risk hurting a friend's feelings.
I also make a point of explaining these methods of mailing occasionally to those who have a bad habit of forwarding countless addresses, although it usually goes unheeded! And, I keep all of my security software up to date, knowing I'm on those folks mailing list! /o:?