An idea that's been kicking around in my head is a widget with an e-ink display for hackerspaces, cafés, and other multi-user spaces that displays the a password-of-the-day along with a qrcode for easy login. Heck, include an NFC chip that hands out application/vnd.wfa.wsc objects as well.

I'm not sure how useful it would be beyond the cool factor, of course … the cafés in my area don't seem to change their wifi passwords often at all, so I assume they're not very concerned about leeching. The typical practice of printing it on a receipt or writing it on the board next to the soup-of-the-day is probably hard to beat.