I've tried Simple Green, citrus-based degreasers, "purple" degreaser, fancy engine degreaser....and they never seem to really do anything, no matter what I use them on. They smell bad and I'm always left feeling like I don't think they're working.
So, tell me your degreaser success stories - about the thing you needed to clean and the product that made you say, "Wow - nothing except [blank] could have done the job".