So Kevin C. Tofel over at Gigaom has a nice little tutorial on how to factory reset your Chromebook even without your password. Normally, when you are logged in and have a password, you navigate to Settings > Advanced Settings and click the Powerwashed button. That will reset your device back to its factory settings getting rid of everything and making it like it was right out of the box. I guess the first question is why on earth would you want or need to do this in the first place (aside from forgetting your password).
In Kevin’s case he had loaned a Chromebook to a friend, he factory reset it so his friend could use his Google account and start fresh. When he got the Chromebook back his friend had not factory reset it back. So this left Kevin with access to Guest Mode only, meaning he could not get into the Settings to perform a Powerwash. After some investigation he discovered a method to factory reset without a password. At the first login screen, simply press Alt+Ctrl+Shift+R and tap the Restart button. This brings up the Powerwash prompt, where can choose to wipe the device clean within a few minutes.