Backpack Dry Bag liners work reasonably well. You just put one into your backpack and put your gear into the dry bag then use the roll closure to make reasonably waterproof.

http://www.outdoorkit.co.uk/product.php?product_id=2464

http://www.nomadtravel.co.uk/catalog/view/ortlieb-rucksack-liner-and-dry-bag-

Waterproof back pack covers works quite well at keeping most of the rain of the back pack as well.

http://www.outdoorkit.co.uk/product.php?product_id=2454

Reversible Camo/Hi vis waterproof back pack covers are quite useful.

http://www.wildlifewatchingsupplies.co.uk/trade/acatalog/Rucksack_Covers.html

Otterbox will probably have a waterproof case for most small electronic gadgets

http://www.otterbox.com