Protect DC Circuit from too much voltage
One thing not specified by the original question is if they are looking for protection of I/O signals or power supply connections. 

If the protections are needed for I/O signals then the suggested zener diode circuits or other methods including TVS (transient voltage surpressor), clamping diodes are the useful ways toward protection. 

When protecting at the power supply connections there are a number of schemes that can be utilized. The previously mentioned poly fuse or other type that is designed to trip at a voltage overload are workable but often have a kick-in delay time where the circuit is momentarily exposed to the high voltage condition.

My suggested approach is to use a design that uses a wide voltage range input regulator circuit and a polarity protection diode. This gives very reasonable "protection" for voltages that a user is likely to mistakenly apply to the power supply input. Here is a block diagram of the suggested approach.



This approach also becomes the power supply for the device itself. For devices that operate at low power this approach can utilize simple linear regulators such as the 78xx series. These will protect for input voltages of up to 35 volts or so. Higher power devices will want to utilize a wide supply voltage range switcher type regulator. There are various devices voltages from vendors such as TI and National (now actually part of TI) that can support input voltages up to 60V and in some cases even 80V.