I don't know much about transmissions, but this looks like an interesting idea—the "NuVinci" (ick!) Bicycle Continuously Variable Transmission:
I don't know if it's truly a major innovation or a engineering novelty, but I reckon it warrants a bit of a look from folks who actually know something about bike design (as opposed to my complete ignorance).
It's being put to use in a funky new bike by Ellsworth, complete with sexy cruiser-style lines and a super quiet belt drive. They're calling it, in all humility, "The Ride".
[Thanks to the ClogWog for the tip-off.]