One morning a few years ago I was flying westbound at cruising altitude on either an A319 or A320 when we banked fairly sharply to the left. Not quite alarmingly but more sharply than a normal maneuver at cruising altitude, and enough to make me glance out my window to the right. Below and to the right a CRJ went screaming past eastbound, close enough that I could read American Eagle and the tail number. I wish I had made a note of the tail number so I could've gone back and looked it up.