Sit-ups and crunches?
Definitely the abs, but it really can have an effect on many muscles. Assuming you are anchoring your feet, you will effect the quads,the anterior tibialis, probably the iliopsoas, you will use the neck muscles to stabalize your head, etc. I would offer that one of the best ways to really figure this out is to grab your anatomy book and do some sit-ups and crunches, and concentrate on where you are feeling the muscular contractions.