Anybody that has spent time camping knows that job number one (aside from staying alive) is to make sure stuff stays dry. It was drilled into me during my time in the Army and the thought persists with me to this day. I've found that simply relying on ziploc bags or that 'waterproof' chamber of my backpack is seriously lacking.
Nowadays will all the electronics we carry...if you are out on the trail and you get caught in some rain (been there), or have fallen into a creek (also been there), or especially if you spend time in a canoe or kayak, having a dry sack that does it's job is essential. I've found the dry sacks from Outdoor Research to be the most cost effective option on the market today. I've never had one leak and they come in a variety of sizes. I usually carry two...one for electronics, and one for my wallet/keys (gotta protect the key fob!). On backpacking trips I'll add another large bag for clothing and socks as a just in case. I've also used these sacks to haul food and sleeping bags.
Outdoor Research Ultralight Dry Sack
They are made from a super light weight waterproof (duh) 40D silnyl fabric. They utilize the old school and still entirely functional sealing mechanism of rolling the top over itself and using a buckle to secure the top. In my mind, it's hard to see me camping and hiking without these dry sacks. I find it to be one of the most essential pieces of gear that I own, and considering their low price, I recommend them to every person that is an active hiker and camper.
Price: $10 - $45 depending on size
Available from Amazon