I own a Kelty Redwing 3100 that I use for groceries, travel, and pretty much anything non-bike related. I've had it for about 4 years, it's indestructible but it's also so large I can fit a 737 inside of it.

So when I got a bike, I went to REI for something smaller and better for a bike, and got the Novara Commuter Backpack from REI. It fits all of the tools I need, all of the other random stuff I need and a few small things I may pick up during the course of a day. It's laid out better than the Kelty for commuting. It has worked out quite well. It's yellow, but not an obnoxious yellow and it appears to be pretty bomb proof. I stopped at a liquor store on the way back from work and the clerk said "that bag looks pretty durable" as I attempted to shove several large bottles of beer into it. Seems to be the general consensus.