Zener + MOSFET overvoltage protection
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simulate this circuit &ndash; Schematic created using CircuitLab

I've chosen BSS84s only because they had low Vgsth which shows the effect more readily; you'll want to find more suitable devices for M1 and M3 as they carry all the current. Feel free to play with it in CircuitLab, I find it needs some tweaking for any given application.

M1 is for reverse voltage protection. It's intentionally installed "backward" (with the not-shown body diode cathode toward the right side of the circuit). When voltage of the correct polarity is applied, the mosfet is on and M1 is more or less invisible to the circuit. When reverse-polarity voltage is applied, M1 is off and its body diode is also blocking current flow. M1 must be selected to be able to withstand sufficient forward current as well as reverse voltage. For higher voltages you may run into trouble as many common mosfets tend to "spec out" at about 20V or so.

M3 is normally on via R2; R1 biases D1 and the voltage is applied to M4's gate. As the applied voltage rises we get closer and closer to M4's turn-on voltage and it starts to pull M3's gate back up toward the voltage at its drain, shutting it off. You'll have to select D1 such that M4 is fully-on at the desired overvoltage point, and M4 such that it's not starting to conduct too early.

The circuit isn't perfect; I've already mentioned that it's not suitable for voltages approaching 20V because it begins to get difficult to find mosfets which can withstand this voltage; I primarily use this for smaller, low-current battery operated applications where we're in the 3-10V range, where it has performed very well given how inexpensive it is.