Photo by Basheer Tome.
When it's really coming down hard, you probably put your windshield wipers on the highest speed possible. Sometimes this helps, but sometimes it's still hard to see in front of you, especially if it's foggy. Snopes explains why a pair of polarised sunglasses can improve your vision in rain and fog:
Wearing polarized sunglasses when driving in the rain during the day will help a driver see better. Poliarized sunglasses work to block horizontal components of scattered or reflected light, which means they help counteract the scattering of light that atmospheric effects like fog or rain have on daylight.
Note that this is only polarised sunglasses. Non-polarised lenses will just make it darker and harder to see. In addition, this tip doesn't really work at night, either, so it's probably safer to stay off the road if you've got darkness and rain working against you. Hit the link to read more.
Pulling the Shades [Snopes]