I have been using these Outdoor Products dry sacks for about a year now. They have been on several multi-night and day hikes and they have never let me down.
I was pretty skeptical of them at first due to their low cost, so I tested them before I trusted them to keep my stuff dry. I stuffed the bags full of t-shirts and let them sit under running water. The contents remained dry. Then I actually submerged the bags for 2 minutes and the contents still remained dry except for some very small areas where water came in at the closure. I was thoroughly impressed. For use inside of a backpack in the rain and even in quick accidental dunks, these are completely sufficient.
These have turned out to be an excellent way to keep gear dry and keep it organized. The bags are made of light weight rip stop nylon. They are light enough to use as organizers even when you don’t expect rain. The seams are fully taped and the closures do a good job of keeping water out if you follow the directions.
I used to just use a trash bag to keep my things dry in the rain but these smaller bags offer more organization potential. They will only set you back $10 for the 3 pack at Wal-Mart. For the price, you can’t beat them.