I probably carried around more weight than I should have back in undergrad, and after busting a few backpacks (primarily ones with cheap plastic zippers--once they go, they're gone) I got one from Mountain Equipment Coop (if you happen to be in Canada):
http://www.mec.ca/Products/product_listing.jsp?FOLDER%3C%3Efolder_id=2534374302699713
But really, something from a camping supply (or maybe army surplus) should probably suffice. Look for good fit, water-resistance (good for if you get caught in a storm), and side pocket / mesh for your coffee mug / water bottle (or both).
EDIT: If you care about the appearance of your laptop, get a sleeve for it--they get scratched up very quickly rattling around in backpacks. For me, I carried around a second bag with my laptop / adapter in it.