The budget-minded who want to keep their gear dry, use gaffer tape to attach a trash bag to their cameras. For hardcore pros who shoot in rain all the time, Aquatech covers are great. They are made of a waterproof material and are designed so that the condensation isn’t an issue on the gear. They come in all different sizes and have special eye pieces you can buy that fit so you can still see what you are shooting without jeopardizing your camera. Think about all the switches on the top of your camera and the hotshoe, that is where water can easily slip into your system and wreak havoc. I would guess snowy areas would have some applications on the wet snowy days too. Check out a 2003 report on them in PopPhoto.
Since I live in an area that sees no snow and little rain I decided to try something a bit more economical: Storm Jackets camera covers. The Storm Jackets come in different sizes, I have the medium and the large, which are both around $40 each.