The usual method is a comparator, which are designed to switch very fast (faster than your interrupt routine can respond, certainly). You get to set the trigger voltage accurately with a resistor-divider, you can determine how much hysteresis to use, and most comparators will work from 5V upto 30V or so and drive any logic load (but need a pull-up resistor since the outputs are usually open-collector). For example I've recently used an LM339 (quad comparator in 14 pin DIL package, all of 12 pence (20c))
At a pinch an op-amp can be used as a comparator, but its not ideal.