Though the penalty for driving without a registration can vary from state to state, there is no state in which it's legal to drive an unregistered vehicle, or a vehicle that was registered but on which the tags have expired.
Each state has its own specific rules and penalties for dealing with drivers who are ticketed for driving an unregistered vehicle. But they tend to follow the same pattern. Sometimes if a vehicle has been registered but the registration is now expired, the penalty is less than if a vehicle has not been registered at all under its current owner.
A car whose tags have been expired for a short amount of time will usually draw a lesser penalty than one whose registration expired several months ago, for instance. If the registration has been expired for only a few months—some states say less than 6 months—a ticket can be issued but it's one that's considered a non-moving violation, like a parking ticket, and won't affect your driver's license or car insurance rates.
A registration that's been expired for 6 months or more in those states, though, may be treated as a moving violation that can have more serious consequences. If you receive a ticket for no registration, and the vehicle is actually registered, you just don't have proof of registration on you or in the vehicle at the time of the ticket, that's taken into consideration and often once proof is presented to the court the ticket is dismissed or lessened.
Some vehicles don't fall under the law that requires registration. Off-road vehicles, vehicles that have been granted a non-operating license by the state, and vehicles that are currently stored are often not required to be registered by the state.
If you're ticketed for driving a vehicle without registration, and your particular offense is treated as a moving violation, you could face several consequences.
Driving without vehicle registration shouldn't be confused with driving without a license. Driving without registration is when you haven't filed the paperwork and paid the fees to register the vehicle with your state agency, usually the state's Department of Motor Vehicles or whatever the licensing agency is.
Driving without a license is when you operate a motor vehicle without a valid driver's license. This carries its own legal penalties and problems, and is a separate issue entirely.