I'll second the Ergon BC3 (http://www.ergon-bike.com/us/en/product/bc3) recommendation. I have the Ergon BC3--it's a really nice backpack. The pack is held completely off your back (no vented foam padding) and allows for tons of air flow. It has a removable "organizer" (for pens, pads of paper, etc). The "f-link" system they use is pretty neat too, you can shift your weight around (I believe the pack was initially designed for MTBing) and your pack stays put.

I did have a small issue with one of the seams coming up a little, but I gorilla glued it back into place and it's working fine. It's nice because it has a hydration bladder in addition to being big enough to fit just about anything you'd need on a commute. The pack quite rigid though, so if you need to compress it and stuff it somewhere, you may have a hard time (the hip straps are reinforced with plastic). The front has an elastic bottom, so you can stuff extra stuff in there (I used to store my u-lock in it).

It's a great pack unless you're trying to carry a ton of stuff. Most non-messenger cycling bags are relatively small and will typically only fit about a pair of clothes (shirts and pants, maybe shoes if you really cram it), bike tools, and a couple of other small things.

I picked this pack because I wanted something: waterproof, large enough to carry more than just the "bare essentials" (without being a closet like those Chrome backpacks), and hydration compatible. The Ergon BC3 fit that bill, and even though it's expensive (I guess it's not that expensive when you compare it to some of the other packs being posted here), you can find them for about $150ish on eBay if I remember correctly. I've had mine for about a year.

I'm a sucker for trying different things, so I recently picked up a Camelbak's H.O.S.S. bag. I picked it up for pretty cheap (about $70 I think). I've been using it lately instead of the Ergon BC3 just to see how I like it. It's got a waterproof rolltop, a bunch of different pockets/holders in the front pocket, an expandable front "holder" area for a helmet/lock/big things, and a hydration bladder. It's also got a suspension frame, which keeps the pack off your back, which is also quite nice.

I don't like the hose that Camelbak included with their pack (it's shorter than what a typical Camelbak bladder would come with), but that's a cheap fix.

Overall, I'm not sure which I like more. I think I like riding with the Ergon BC3 more because the f-link system works so well, but I like the Camelbak more in terms of storeability (it's much smaller, while having the same storage space--if not more due to the rolltop and less rigid). Not that I really care since I'm on a bike in goofy clothes anyway, but I think the Ergon also looks a little funky off the bike (if I stop to do some quick shopping).