I use panniers on my bikes that have a rack, but I wear a backpack when I ride my bikes that don't.

For about 7 years I used a Minnesota Vikings backpack I got at JC Penney for free. It was on clearance for $10 and I had a $10 off coupon. Score!

When that one wore out I picked up a Swiss Gear laptop backpack on another clearance sale off of BuyDig for $22. Not specifically designed for commuting purposes, but I've found the different sections quite handy. My clothes can go in one pocket, my lunch in another and my wallet/phone/etc can go in the third. It works.

When this Swiss Gear wears out I'll probably seek out another clearance deal. I'm not too picky, but I am cheap.