Computing and Programming.
One of the most fascinating subjects I've found is computer science and programming in particular. Its indeed awe inspiring to watch a machine taking rational courses of action and performing a given task according to the programmers needs in a flexible, precise and exact way.
Even more impressive is the newer development on such fields as Artificial Intelligence and Reconfiguring Computing which grants us such awe inspiring things as computers capable of adapting according to stimulus from the environment and systems capable of learning and making decisions using human like logic known now as weak AI.
Right now I'm working on computer vision and speech recognition which is something that couldn't be conceived several years before.