Well there's Alex Flinn. I'm not really sure if she's considered as "undiscovered", since one of her books have already been made into a movie, which is called Beastly. Venessa Hudgens is in it. I'm not sure if there are more movies made from her other books, since Beastly's the only book I've read. But it's really really good. The story might be a little cheesy, so a lot of people may not like it. But I don't mind cheesy.
And then there's Richelle Mead. She's not exactly undiscovered either, since she's already the author of a bestseller series, Vampire Academy. But I still think she's not appreciated enough. I liked Vampire Academy much more than every other vampire book I've read and I really think Vampire Academy deserves to be more popular. It's even better than The Vampire Diaries, in my opinion. But it's probably not gonna happen until a movie or TV series gets made out of it.