I rotate through a lot of different glove and glove combinations based on the conditions at hand. For true waterproofedness, I keep a pair of these in my bag all winter and also if heavy rain is forecast:

Outdoor Research Mt. Shuksan Shell Mitts ($53)
These Gore-Tex mitts are light, rugged, stuffable, and totally waterproof. The palms are made of 1000d Hydroseal-200-coated Cordura and are waterproof to over 200 PSI.