Yubikey is sort of like this. It's a USB key with a small contact on it. It reports to the OS as a keyboard and when you press the contact is inputs a very long string of characters followed by an ENTER. So at the 2FA page you just press the USB key button and you get logged in.
I actually have two around my neck at the moment, I use them for my own hand-rolled websites. They're concienient, but just being able to push a button in an app would be even more so. Not to mention, the backend code they supply on their website is anything but pleasant.
Mt. Gox uses Yubikey for their 2FA.