When I commuted in the 70s/80s, I always carried a cape - rolled up tight, secured with a spare toestrap, and tied to the seatpost.  It was yellow plastic, with a simple gusset and press-stud at the neck, and I wore a sou'wester with it, being the days when only racers wore helmets.  It was brilliant for keeping the rain off and (rather masochistically) I used to love riding along in a downpour in my own personal tent.  I got far less wet than I feared I would, although feet and legs suffered.  I'm in Wales now, and planning to commute through the winter, so I need to think about weather protection.  I'm very tempted by another cape ... (although not paying North of £70 for one)