I'm going to repeat what others have said: Orlieb. I use the front roller classics on my back rack. In addition to being incredibly rugged and very easy to put on and take off, they are totally waterproof. I have ridden through torrential downpours with expensive electronics in them and never had so much as a drop of water get through. The front rollers hold a surprising amount of stuff. I usually just take one with my clothes, lunch, and iPad in it. If I need to bring the laptop, extra books or papers, or whatever, then I'll use both.