I've ordered a ShareRoller to use as a motor so my wife and I could share it on our Bromptons.

It's a small 5.5 lb rectangular box that opens up to run a friction motor over the front wheel - the base version has a 12 mile radius maxing out at 18mph, with small lights and a usb charger built in.

The most prohibitive thing may be the cost - about US$1,000 for the base model but I love the idea of being able to remove it easily (less than 10 seconds to install/uininstall) and use the Brompton as per normal.