You can have a death of a major character happen, which does a few things. It takes any idea that the reader had of where the story was going and makes them reevaluate the situation. It can also be a good way to get the reader emotionally attached because they never know when someone that they got to know could be taken away from them. However, the downside is that if the character was their favorite character, you risk alienating the reader so much that they don't want to finish the book.
You can have a secret be revealed, such as a long lost daughter, or relative. This works because its surprising, and powerful if you hint at it before it happens. The downside is that if not done correctly, you can cause the reader to stop right there.
There are a ton of different ways to implement plot twists, and the key is doing it correctly. You can't just surprise the reader with new information without leading up to it. You can't break the own rules of your world for the sake of surprising the reader. So plot twists are necessary, but also need to be managed correctly for them to be pulled off. If you fail, your entire book will fail.
What are some of your favorite plot twists?
What sort of twists have you implemented in your book?