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=2464http://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=2454Reversible Camo/Hi vis waterproof back pack covers are quite useful.
http://www.wildlifewatchingsupplies.co.uk/trade/acatalog/Rucksack_Covers.htmlOtterbox will probably have a waterproof case for most small electronic gadgets
http://www.otterbox.com