Computer science: It's certainly not MIT, but we have some amazing professors and opportunities for undergrad research. I'd say the strongest research groups are the Systems one and the one led by Ken Forbus (those guys work on some sort of machine learning stuff--that's pretty much AI + cognitive science).
Overall, there's been a lot of progress in the comp.sci. part of the Electrical Engeneering and Computer Science department for the last couple of years.
Given that and our econ department, Northwestern makes a good choice for double majoring in econ and comp. sci. or majoring in one and minoring in the other.