We covered the five accent/graphical marks in French today. Le tréma (naïf, Noël, Israël, canoë, coïncider) gets me excited, because it gives me a chance to say diaeresis in class (kids remember these things!). I also dig le tréma because it appears in English as well and thus provides at least a semi-familiar example to show students the exceptional vowel pronunciation the symbol indicates. The obvious example—perhaps the only familiar example— is naïve. But I still recall reading an old edition of some novel and encountering coöoperate... or was it reëlection?
I got to wondering if I could find a list of English words that use the trema. Here are a few from Wikipedia: