Wich programs are big on rotators?

I think your question will be hard to answer. Most programs are likely big on rotators. There are probably some programs who dont weigh rotating at their program as heavy as others and if you do a search on this site you should find a number of posts with opinions, accurate or not, on that issue. It seemed to me after last year that smaller programs tended to be a little more rotator heavy which makes I guess makes intuitive sense since the impact of one bad egg has a much larger effect than at bigger programs. However, if you are asking the question because you are just trying to increase your chances of matching then you still might be better served at a larger program since they obviously have more spots for which you can fight.

If you want helpful responses to your post, you might be a little more specific on what you are looking for in a program (region, type, etc.), name a couple of specific programs you are considering, or at least explain why you are asking the question. Right now your post is very vague and that will probably make it difficult for anyone to give you any helpful information.

I have heard that Northwestern won't interview without a rotation. Does anybody know whether this is true or not?

Northwestern will interview non-rotators. However, not many....and the ones they interview are typically better applicants. On average, the ratio of rotator to non-rotator interviewees is about 3:1. But if you didn't rotate, and do get an interview, your odds of matching there are pretty good, since they usually match 1-2 non-rotators a year.