Can users sign up for emails when people respond to a question?

Of course yes, you can see the subscribe button just beside the submit answer button. Try yourself.

Note: By default users who post question will be notified when new answer is submitted, if it is enabled in the configuration.

Note an answer but another question: Can I view all questions/catogories that I am subscribed to?

Presently not. Subscriber management feature is due in v2.0 (next major release).