The dictionary lists испугать and напугать (and one gives пугнуть) for the perfectives of пугать. Is there any difference in meaning? Is one more frequent than the other(s)?
Испугать and напугать are synonyms. They are mostly interchangeable. The difference if exists is very subtle.
напугать = to make shocked
испугать = to make concerned
One difference is when using the words with reflexives. It is possible to испугаться за (for) something but not напугаться за (for) something:
Когда сына призвали в армию, я испугался за его жизнь = When my son was conscripted to the military, I became concerned for his life.
Using напугался would be incorrect here.
More subtle difference is that напугать can more often than испугать be used sarcastically in the sense "to make shocked".
Когда я его брал на работу, его уровень знаний меня напугал. = When I was considering him for a position, his level of knowledge shocked me.
This means the knowledge was either very good or very bad. Conversely испугать is more used to mean "to make concerned":
Когда я его брал на работу, его уровень знаний меня испугал. = When I was considering him for a position, his level of knowledge made me concerned.
This means that I possibly have serious questions on where he got all this knowledge (may be he is a foreign spy?) or may be I became concerned for the education system that makes such bad graduates.
Пугнуть has a different meaning, it means making a gesture to frighten somebody or (by implication) any other short-term action with that purpose.
By the way, you can infer the meaning from the very prefixes themselves.
Prefix на- means "to do plenty, a lot of something", as in words настрогать, наложить, напилить, наделать, навонять, напустить.
So напугать means "to frighten a lot".
Prefix ис- means "to do something completely, make permanent change to its logical end, to the final target, reaching the aim", as in words изрубить, испепелить, измотать, изжить, исправить, исписать, изрисовать (compare English words extinguish, exterminate, explain with Latin prefix ex- which is cognate with Russian iz-/is-).
So the meaning of the word испугать means "to frighten completely", "to frighten so much that the object changes their behavior, becomes concerned, more cautious".
I'd like to take a more historical approach to explain it. The real difference is in the fact that one word is more colloquial, day-to-day, and the other is higher register: the word испугать features the Old Church Slavonic prefix ис- and therefore has a higher register connotation.
The answers people gave here are pretty correct to some extent, however what they're doing is trying to generalize random consequences of the real cause: low vs. high register.
Just like an example above: Меня напугала собака -- a dog scaring someone in the streets, is a more down-to-the-earth situation, so the low register East Slavic на- is used.
Меня испугала перспектива провала -- "the prospect of failure scared me" -- this situation is more metaphorical, more abstract, therefore the higher-register ис- from Church Slavonic is used. A dog can испугать as well, but that would mean, in this context, something more serious, than the less serious напугать (more serious vs. less serious stems from high vs. low register as well)
It's often useful to do a Google image search to help with differences between closely related verbs. Doing so won't help with grammar, but for meaning it's helpful. Search for напугать и испугать and see...
на also means up, onto, to,—so one could say напугать means "putting a sudden fear on someone"—it's not correct in grammar, but so what :)
настроить can mean to build a lot or a certain amount of X, but one can also think of it as building (tuning, etc) something up (onto, to) a certain point. To tune a TV to a channel, a person against (towards a state that is against) me etc.