signature quote

A signature quote is a quote that can fit on one or two lines, so you can add it as a part of your signature block after your (user) name to the end of an e-mail message, Usenet article, or forum post.
Basically the equivalent of wearing a t-shirt with a slogan or having a bumper sticker attached to your car.

"I just love your signature quote."
"What's-his-name has the coolest signature quote."

"That has to be the coolest signature quote ever!"
"Don't you just hate sappy signature quotes?"