Here’s an Instructable from “techno guy” with a misleading title. It teaches you how to set-up the Pi to start up from a USB drive which contains the main operating system image. Granted, you’ll still need the SD card in the Pi to actually ‘boot’ the Pi, but after that it’s all driven from the USB drive. You’ll need a separate Linux machine to manipulate the OS images and suchlike. Read more here.