In the Lupfantomo article in Vikipedio I found a use of 'kiu' that confuses me:

En la folkloro, lupfantomo estas homo kiu havas la povon iĝi lupo.

I thought 'kiu' only meant 'which' in a question sense. Is it often used as a preposition (if that's how it's being used here)? Are there other 'ki-' correlatives used like this?

edit: Would 'ke' be more appropriate where 'kiu' was used in that sentence?

I thought 'kiu' only meant 'which' in a question sense.

No. Correlatives create relations (hence the name) between two objects in different sentences (this isn't the best explanation, I know...). In Esperanto they are also used in questions, as you say. In some languages (Finnish comes to mind) those two functions might be separated.

En la folkloro, lupfantomo estas homo kiu havas la povon iĝi lupo.

You could think of it as: "En la folkloro, lupfantomo estas homo. Tiu homo havas la povon iĝi lupo."

Are there other 'ki-' correlatives used like this?

Sure! First example to cross my mind: "Ĉio estis trankvila kiam mi revenis al hejmo".

Would 'ke' be more appropriate where 'kiu' was used in that sentence?

No. "ke" introduces an objective sub-sentence. The meaning of "ke" is "what follows is the object" (or sometimes the subject).

"Multaj opinias ke Esperanto estas facila lingvo"
In this sentence, "Esperanto estas facila lingvo" is the direct object of "opinias".

"Plaĉas al mi ke vi helpis min".
In this sentence, "ke vi helpis min" is the subject of "plaĉas".

Oŝo-Jabe:I thought 'kiu' only meant 'which' in a question sense.

It can also mean "who", which is how it is being used in your example, almost exactly the same way that it would be used in English.

"In folklore, a werewolf* is a person who has the ability to become a wolf."

In the English translation, "who" is not being used as a question word, even though in English it often is. It's basically the same thing in Esperanto.

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* I assume "lupfantomo" means werewolf - it isn't in the vortaro, so I'm just guessing.

The ki- words can also be used as relative pronouns/adjectives/adverbs, which is how "kiu" is used in that sentence.

If you don't know what a relative pronoun is, the wikipedia article is very clear about it.