I'm surprised no one mentioned the United College Plus card specifically for college students. No annual fee, but you only rack 1 mile per $2 spent. Only useful if you fly on United to go home, though, which I did when I was a college student.

I told my sister about that card, so she applied and received that card, too. Although she is no longer a student, she continued with that card and was automatically upgraded to a United Signature card--with no annual fee. Meanwhile, I cancelled the card when it was no longer useful and reapplied when I was back in graduate school. Not sure if I'll keep the card, but I figure that as long as there is no annual fee, I'll hang on to it for a while.