The environmental friendly YubiKey is minimal sized (3 grams, 3 mm thin),
battery-free and waterproof. It survives going through the laundry and
being run over by a car. With extra hard gold on the USB-contact it will
last for daily use on a key chain for 10 years.
They offer a free cloud based authentication service. And, free open
source authentication server software if you don't want to use their
"You can see a lot by just looking."
-- Yogi Berra