Short commute of 3-6 miles? Anything really. An older road bike, english 3 speed,mountain, hybrid, etc.

Medium 10-15 miles? Older road bike with provision for fenders/larger tires, cyclocross bike,

20+ miles? Most folks I see with longer commutes are using cyclocross, touring, or higher-end road bikes.

It really depends on what weather you ride in and what you value when riding. You can commute on any bike you want - don't listen to other people, including me.