Not really a bad one, but it still made me laugh...
Customer calls in with a question about his email...
customer: "how much space do i have on my email address"
me: "10 megs"
customer: "ok, so how many gigabytes is that?"
me: mute button on "hahahahaha"
me: mute button off "um, thats about 1/10,000th of a gigabyte"
customer "oh, ok, bye"