M1 Sedan ONE
Credit: M1-Sporttechnik, Tom Gonsior

In most urban areas, one gear is all you need to get where you're going, but when you run into a bridge or small city hill, a bit of an electric boost is helpful. That's the thinking behind the M1 Sedan, a German carbon fiber single speed with a rear wheel motor that's great for small hills, but not, say, a San Francisco-grade slope. At under 33 pounds, it's one of the lighter e-bikes out there, and you easily remove the motorized rear wheel and swap in a regular one when you want to ride without a boost.
[$3,756; m1-sporttechnik.com]